WALLACE  ON  THE  EYE. 


TREATISE  ON  THE  EYE. 


DISCOVERIES  OF  THE  CAUSES  OF  NEAR  AND  FAR  SIGHTED- 
NESS,  AND  OF  THE  AFFECTIONS  OF  THE  RETINA, 
WITH  REMARKS  ON  THE  USE   OF  MEDI- 
CINES AS  SUBSTITUTES  FOR 
SPECTACLES. 


BY  WILLIAM  CLAY  WALLACE, 


CONTAINING 


OCULIST. 


SECOND  EDITION. 


NEW  YORK: 


PUBLISHED  BY  SAMUEL  COLMAN, 


VIII  ASTOR  HOUSE. 


1839. 


Entered  according  to  Act  of  Congress,  m  the  year  1839, 

By  WILLIAM  C.  WALLACE, 

the  Clerk's  Office  of  the  District  Court  of  the  Southern  Dis- 
trict of  New- York, 


RECOMMENDATORY  NOTICES. 


From  Sir  John  F.  W.  Herschel. 

"  Highly  curious  and  interesting,  as  well  as  in  many  respects 
new." 


From  Professor  Knight,  of  Yale  College. 

"  He  has  made  interesting  discoveries,  which  throw  much  light, 
on  hitherto  obscure  points  in  the  physiology  of  vision." 


From  the  Report  in  the  London  AthencEum,  of  the  eighth  meeting 
of  the  British  Association  for  the  Advancement  of  Science. 

"  Sir  David  Brewster  laid  before  the  section,  a  series  of  beautiful 
preparations  of  the  eye,  made  by  Mr.  Wallace,  an  able  oculist  in 
New  York,  calculated  to  establish  some  important  points  in  the 
theory  of  vision.  He  stated,  'that  one  of  the  most  important  re- 
sults of  Mr.  Wallace's  dissections,  was  the  discovery  of  fibres  in 
the  retina.  Sir  John  Herschel  had  supposed  such  fibres  to  be 
requisite  in  the  explanation  of  the  theory  of  vision,  and  it  is  there- 
fore doubly  interesting,  to  find  that  they  have  been  actually  dis- 
covered.' " 


From  Professor  Silliman. 

"I  have  been  very  favourably  impressed  by  his  various  commu- 
nications on  the  eye,  and  topics  relating  to  that  organ." 


From  the  London  British  and  Foreign  Medical  Review. 
"  This  is  an  ingenious  and  well  written  paper." 


Digitized  by  the  Internet  Archive 
in  2013 


http://archive.org/details/treatiseoneyeconOOwall 


PREFACE. 


On  a  subject  on  which  so  much  has  been  writ- 
ten, and  which  has  been  so  often  investigated,  it 
might  be  thought  that  nothing  new  could  be  ad- 
vanced ;  yet  the  philosophical  enquirer  will  per- 
ceive that  I  have  not  only  availed  myself  of  what 
has  been  already  written,  but  that  I  have  added 
many  new  facts  which  explain  much  of  what  was 
formerly  obscure,  in  the  physiology  of  vision. 

90  Chambers  Street,  New  York. 

July  Uh,  1839. 


LIST  OF 

Pig. 

1  Eye-socket  of  Halibut^ 

2  Halibut, 

3  Eye-socket  of  Turtle, 

4  Bramah  Press,  &c. 

5  Plan  of  Eyelashes, 

6  Muscles  of  Eyelids, 

7,  8,  9  Eye  of  Owl  and  Plan, 
10,  11       "  Rhinoceros, 
12  "  Lobster, 

13,  14  Apparatus  for  the  Tears, 

15  Illustration  of  do. 

16  Muscles, 

17  Plan 

18  Dark  Chamber, 

19  Section,  '  "  - 
20,  21  Plans, 

22  Fibres  of  Magnifier, 

23  Strainer  of  Halibut, 

24  Fibres  of  Retina, 

25  Bloodvessels  of  do. 

26  Illustration  of  Inverted  Image, 
28,  29  Pupils  of  Skate  and  Cat, 
30,  31,  32,  33,  34,  35  Plans, 

36,  37,  38,  39,  40  Striped  Bass, 
41,  42,  43  Halibut, 

44  Adjuster  of  Ox, 

45  Eye  of  Sheep,  J 

46  "  Man, 

47  "  Ox, 

48  Sheep, 

49  Ciliary  Veins, 

50  Plan, 

51  Bloodvessels, 

52,  53  Eyes  of  Lynx  and  Eagle, 

54  Choroid, 

55  Portion  of  Network, 


CUTS. 


Original 
Yarrell 
Original 
Original 
Original 


Original 


Original 
Soemmering,  Sen. 

Original 


Original 
Original 

Soemmering,  Jun. 

Original 
Roget 
Original 
Original 

Soemmering,  Sen. 

Original 
Original 
Original 
Original 
Original 
Bauer 
Original 
Original 
Original 
Original 

Soemmering,  Sen. 

Original 

Soemmering,  Sen. 

Soemmering,  Jun. 

Soemmering,  Sen. 

Original 


STRUCTURE    OF   THE  EYE. 


No  person  of  ordinary  intelligence  would  on  the 
inspection  of  a  steam-engine,  acknowledge  that 
the  parts  of  a  machine  of  such  power,  and  indica- 
ting so  much  thought  and  design,  could,  by  nat- 
ural causes  alone,  assume  their  form  and  be  placed 
in  situations  proper  for  the  purpose  for  which  they 
were  intended.  It  is  clear  that  without  the  smel- 
ter the  ore  might  have  remained  in  the  mine  for 
ever,  and  that  without  the  mechanic  the  metal, 
when  formed,  would  never  have  been  a  steam- 
engine. 

It  is  proved  by  geologists  that  the  earth  we  in- 
habit was  at  one  time  a  melted  mass,  of  such  a 
temperature,  that  no  living  thing,  as  at  present  or- 
ganised, could  then  exist ;  there  was  consequently, 
a  period  when  the  machinery  of  animated  beings 
was  formed  and  put  together,  and  when  all  the 
contrivances  we  witness  were  planned  and  execu- 
ted.   There  were  no  gradual  advances  to  perfec- 


10 


STRUCTURE  OF  THE  EYE. 


lion ;  every  organised  being  was  at  once  adapted 
to  the  element  in  which  it  was  destined  to  Hve. 
From  the  examination  of  the  remains  of  animals, 
we  find  that  their  organs  were  as  perfect  thousands 
of  years  ago  as  they  are  now,  whereas  the  master- 
pieces of  human  contrivance  are  daily  undergoing 
improvement.  In  the  construction  of  living  crea- 
tures there  is  no  room  for  improvement  —  there  is 
no  science  nor  art  of  which  advantage  is  not  ta- 
ken ;  for  when  we  discover  a  new  principle,  or  ap- 
plication of  a  principle,  we  find  on  an  appeal  to 
nature,  that  it  has  been  known  and  acted  upon 
long  before.  As  an  example;  —  the  eye  of 
the  halibut  is  directed  upward,  and  the  animal 
could  only  see  in  that  direction  if  there  were  not 
a  provision  for  turning  it  forward.  Below  the  eye- 
socket,  and  communicating  with  it  by  an  opening, 


Fig.  l.-EYE  OP  HALIBUT. 
a  Aperture  between  the  Eye-Socket  and  the  Cavity  beneath  It. 


EYE   OP  HALIBUT. 


11 


there  is  a  cavity  containing  water  which  may  be 
forced  into  the  socket,  and  be  squeezed  back 
again  when  required.  The  skin  is  firmly  fixed  be- 
fore, while  it  is  loose  behind,  and  permits  the  or- 
gan to  be  turned  round  and  elevated,  so  that  the 
animal  can  see  straight  forward  when  the  water  is 
forced  into  the  socket.  To  protect  the  nerve  of 
vision  from  injury  arising  from  the  changes  of  tem- 
perature thus  produced,  a  thick  coating  of  jelly,  a 
bad  conductor  of  heat,  is  placed  between  the  fluid 
and  the  nerve.  (See  Fig.  41.  b.)  The  want  of 
mobility  in  the  neck  is  thus  supplied  by  means 
quite  as  efficient,  and  which  man,  with  all  his  in- 
genuity, did  not  discover  or  apply  to  a  practical 
purpose  for  more  than  five  thousand  years. 


12 


STRUCTURE   OF  THE  EYE. 


Fig.  3. -EYE  OF  TURTLE. 
a  Air  Cavities  in  Eye-socket  of  Turtle. 


The  socket  of  the  turtle  contains  cavities  into 
which  air  may  be  forced  and  the  eye  be  blown 
out,  while  numerous  intersecting  bands  limit  the 
extent  of  protrusion  and  keep  it  from  bursting. 
By  squeezhig  out  the  air  the  eye  may  be  sunk  so 
fa,r  that  there  is  no  danger  of  its  being  injured  by 
striking  against  the  shell,  when  the  head  is  drawn 
rapidly  back. 

It  is  difficult  to  perceive  how  these  objects 
could  be  accomplished  in  any  other  way  than 
above  stated.  If  the  eye  were  pushed  out  by  the 
direct  apphcation  of  muscular  power,  the  muscles 
for  the  purpose  would  occupy  so  much  of  tlie 
socket  that  there  would  be  no  room  for  those 
which  move  the  eye  in  different  directions.  Al- 


HYDROSTATIC   APPARATUS.  13 

though  deservedly  vaunted  as  a  modern  discovery, 
the  principle  of  the  hydrostatic  press  was  under- 
stood and  practically  applied  long  before  the  time 
of  Bramah. 


2 


14 


STRUCTURE   OF   THE  EYE. 


"  A  striking  evidence  of  design  in  the  position 
of  the  eyes  is  this,  that  they  are  so  placed  as  to 
look  in  the  direction  in  which  the  legs  move  and 
the  hands  w^ork.  It  might  have  happened  very 
differently  if  left  to  chance.  There  were  at  least 
three  quarters  of  the  compass  out  of  four  to 
have  erred  in.  Any  considerable  alteration  in  the 
position  of  the  eye  or  the  figure  of  the  joints,  would 
have  disturbed  the  line  and  destroyed  the  alliance 
between  the  sense  and  the  limbs."* 

Another  evidence  of  design  exists  in  the  direc- 
tion of  the  eyes  of  carnivorous  animals  and  of 
those  on  which  they  feed.  The  eyes  of  the  for- 
mer are  directed  forward  that  they  may  observe 
their  prey,  w^hile  those  of  the  latter  are  directed 
backward,  that  they  may  escape  pursuit. 

To  protect  an  organ  so  essential  to  existence,  it 
is  placed  in  a  socket  of  bone  which  defends  it  on 
every  side,  and  projects  above  like  a  roof,  having 
its  edge  furnished  with  the  eyebrow,  which  is  so 
shaped,  that  it  conducts  the  sweat  of  the  forehead, 
or  the  rain  that  may  fall  on  it  away  from  the  eye. 
The  eyelids  are  lined  by  a  soft  moist  skin,  which 
when  they  are  moved,  passes  over  the  transparent 
window,  and  keeps  it  clean  and  polished.  This 
skin  is  turned  from  the  eyelids  to  cover  the  white 


♦  Paley. 


EYELIDS. 


15 


of  the  eye,  and  thus  forms  a  fold,  which  prevents 
motes  from  getting  behind  the  eyeball,  and  de- 
stroying the  organ,  by  producing  inflammation .  (Fig. 
19,  z.)  The  nerve  which  animates  the  lachrymal 
gland  is  spread  out  on  this  skin,  and  there  is  such 
a  sympathy  between  them  that  the  moment  a  par- 
ticle of  dust  irritates  the  sensible  lining,  there  is  a 
gush  of  tears  to  wash  it  away. 

On  the  inside  of  the  eyelids  there  are  a  number 
of  little  glands,  (Fig.  13,  c,)  which  prepare  an  oily 
fluid  which  passes  through  holes  (d)  at  the  edge, 
for  the  purpose  of  keeping  them  from  sticking 
together,  and  preventing  the  tears  from  running  on 
the  cheek,  just  as  water  does  not  pass  readily  over 
a  vessel,  the  edge  of  which  is  smeared  with  oil. 
The  eyelashes,  which  are  placed  in  irregular  rows 
on  the  outer  edges  of  the  eyelids,  serve  as  a  pali- 
sade to  exclude  dust  and  unnecessary 
light.  When  applied  to  each  other 
they  form  an  interlacement  which 
keeps  the  opening  secure. 

Muscles  of  Eyelids. — The  upper  eyelid  is  open- 
ed by  a  muscle  which  rises  at  the  bottom  of  the 
socket  and  is  fixed  into  the  gristle  of  the  eyelid. 
It  resembles  the  other  muscles  of  the  body  in  be- 
ing occasionally  palsied,  and  then  the  suff'erer  is 
unable  to  open  the  eye.  "  With  much  compas- 
sion," says  a  religious  philosopher,  "  as  well  as 


14  STRUCTURE  OF  THE  EYE. 


Fig.  6.-MUSCLES  OF  EYELIDS. 


astonishment  at  the  goodness  of  our  loving  Crea- 
tor, have  I  considered  the  sad  state  of  a  gentleman, 
who,  as  to  the  rest,  was  in  pretty  good  health,  but 
only  wanted  the  use  of  these  two  little  muscles 
that  serve  to  lift  up  the  eyelids,  and  so  had  almost 
lost  the  use  of  his  sight,  being  forced,  so  long  as 
the  defect  lasted,  to  lift  up  his  eyelids  with  his 
own  hands." 

The  eyelids  are  shut  by  a  muscle  that  surrounds 
them,  the  fibres  of  which  draw  them  together  with- 
out wrinkling,  because  they  are  kept  firm  by  the 
gristle  of  the  eyelid.  When  we  close  the  eyelids 
the  pupil  is  turned  upward  :  if  we  place  a  finger 
over  one  eye,  and  wink  with  the  other,  the  eye 
Avill  be  felt  to  roll  on  each  motion  of  the  lid.  In  a 
person  who  cannot  shut  the  eye  in  consequence  of 
palsy  of  the  muscle,  or  the  contraction  produced 
by  a  burn,  we  can  see  the  transparent  window 


EYELIDS. 


17 


raised  beneath  the  upper  eyelid,  and  its  surface 
wiped  clear  at  the  usual  time  of  winking. 

The  velocity  with  which  the  eyelids  move  at 
the  appearance  of  danger  is,  indeed,  wonderful. 
Gunpowder  is  often  exploded  in  the  face,  without 
the  eyes  being  in  the  least  affected.  The  eyelids 
present  themselves  to  the  danger,  and  are  often 
seriously  injured  while  attempting  to  preserve  this 
important  organ. 

The  eyelids  of  the  chameleon  are  drawn  to  a 
small  aperture  opposite  the  pupil,  and  they  move 
with  the  eyeball,  so  that  its  glistening  is  not  expo- 
sed. Without  eyelids  to  correspond  with  its  hab- 
its of  concealment,  its  colour  changing  with  sur- 
rounding objects,  and  its  slow  and  cautious  motion 
would  pass  for  nothing,  if  the  insects  upon  which 
it  feeds  perceived  the  brightness  of  its  eye  ;  but 
like  the  leaves  around  it,  without  even  the  eye  ex- 
posed, it  approaches  its  prey  with  so  little  appear- 
ance of  life  or  motion,  that  the  insect  is  not  aware 
of  the  presence  of  an  enemy  before  it  is  secured. 

Third  Eyelid. — The  eyes  of  birds  are  much  ex- 
posed during  their  rapid  movements  among  the 
branches  of  trees.  To  suit  their  necessities  they 
have  a  third  eyelid,  which,  when  drawn  over  the 
eye,  is  an  effectual  protection  to  the  organ  by  its 
toughness,  and  owing  to  its  partial  transparency  vis- 
ion u  not  altogether  obscured.    It  is  moved  by 


18 


STRUCTURE  OF  THE  EYE. 


Fig.  8. 


Figs.  7  and  8 —MUSCLES  OF  THIRD  EYELID  OF  OWL. 

a  The  Square  Muscle  through  the  edge  of  which  the  Tendon  c,  of 
the  Pyramidal  Muscle  b,  passes  and  plays  over  the  pulley  d. 

two  flat  muscles,  which  having  no  room  elsewhere, 
are  closely  appHed  to  the  back  of  the  eyeball.  One 
of  the  edges  of  the  broader  muscle  resembles  a 
string  case,  through  which  passes  the  tendon  ot 


EYELIDS. 


9 


cord  of  the  other  muscle,  which  is  fixed  to  the 
membrane.  In  some  birds  there  is  a  hook  at  the 
side  of  the  eye.  When  the  muscles  act  they  pull 
the  cord  over  this  hook,  and  draw  the  membrane 
across  the  eye,  as  we  would  hoist  the  sail  of  a  ship. 
No  other  contrivance  in  the  same  space  could 
cover  so  much  surface  with  the  same  rapidity. 


2D 


STRUCTURE  OF  THE  EYE 


Pig.  lO.-EYE  OP  RHINOCEROS. 

As  the  eyes  of  quadrupeds  are  not  defended  by  a 
projecting  brow  like  that  of  man,  they  have  a  third 
eyehd  called  the  haio,  which  in  some  respects  re- 
sembles the  corresponding  membrane  in  birds.  A 
gland  on  the  internal  surface  prepares  a  gummy 
fluid,  which  the  animal  sweeps  across  the  eye  to 


HAW. 


21 


entangle  the  dust  that  falls  upon  it,  and  to  keep  the 
window  moist  and  transparent. 

In  the  rhinoceros  this  gland  is  of  enormous  Size 
when  compared  with  that  in  other  animals.  When 
ploughing  the  ground  with  its  horn,  and  throwing 
earth  and  dust  on  its  enemies  by  way  of  defence  ; 
the  eye  is  much  exposed  and  requires  unusual  pro- 
tection. 


22 


STRUCTURE   OF  THE  EYE. 


Fig.  12.~EYE  OF  LOBSTER. 

a  Brush. 

The  eyes  of  fishes  being  bathed  and  kept  trans- 
parent by  the  fluid  in  which  they  Uve,  have  no  oc- 
casion for  eyehds,  yet  the  shark,  which  is  obhged 
to  fight,  has  a  scaly  covering  which  he  can  draw 
over  the  eye  to  protect  it  when  injury  is  threat- 
ened. 

The  lobster  and  the  crab,  which  are  usually 
found  at  the  bottom,  and  thus  liable  to  have  their 
eyes  obscured  by  sand  and  mud,  are  furnished  with 
a  brush  with  which  they  can  sweep  away  the  im- 
pediments to  vision. 

The  gland  for  preparing  the  tears  is  about  the 
size  of  an  almond,  and  sunk  into  a  hollow  of  the 
bone  at  the  upper  part  of  the  socket,  to  be  out  of 
the  way  of  the  motions  of  the  eye.  The  fluid 
which  it  prepares  passes  to  the  inside  of  the 
upper  eyelid  by  seven  pipes,  so  small  that  they 
will  not  admit  a  hair ;  it  is  prevented  from  running 


APPARATUS  FOR  THE  TEARS. 


23 


— c 


Fig.  IS. 


Pig.  14. 

Figs.  13.  and  14.-APPARATUS  FOR  THE  TEARS. 

a  The  Gland  that  prepares  the  Tears:  b  The  Passages  by  which 
they  are  poured  out  on  the  inside  of  the  Eyelid  ;  c  The  Meibo- 
mian Glands,  which  prepare  the  fluid  for  preventing  their  run- 
ning over  on  the  cheek;  d  The  openings  of  the  Meibomian 
Glands;  e  The  Points  which  take  up  the  Tears  ;  /  The  Chan- 
nel through  which  they  pass  to  the  Nose;  g  The  Roots  of  the 
Eyelashes ;  h  The  Iris. 


24 


STRUCTURE   OF   THE  EYE. 


over  the  edge  of  the  eyelids  by  the  oily  fluid  of 
the  glands  formerly  mentioned,  and  is  collected  at 
the  inner  corner  of  the  eye,  from  which,  unless  too 
abundant,  it  is  conveyed  away  by  the  action  of  a 
muscle  that  enlarges  the  size  of  the  tear-bag, 
and  pumps  into  it  the  collected  tears  through 
two  little  pipes,  the  openings  of  which,  at  the 
inner  corner  of  each  eyelid,  are  kept  in  situa- 
tion by  a  red  substance,  made  elastic  by  fine  hairs. 
From  the  tear-bag  they  pass  to  the  nostril,  and  are 
evaporated  by  the  current  of  air  which  is  always 
passing  over  it  during  the  process  of  breathing. 
"  Can  any  pipe  or  outlet,"  says  Dr.  Paley,  "  be 
more  mechanical  than  this  is.  It  is  easily  per- 
ceived that  the  eye  must  want  moisture,  but  could 
the  want  of  the  eye  generate  the  gland  that  pro- 
duces the  tear,  or  bore  the  hole  by  which  it  is  dis- 
charged.   A  hole  through  a  bone  !" 


EYEBALL. 


25 


Fig.  16.-MUSCLES  OP  EYEBALL. 

a  Muscle  which  lifts  the  upper  Eyelid ;  b  Upper  oblique  Muscle 
passing  through  its  pulley;  c  Lower  oblique ;  Upper  straight 
Muscle;  /  Lower  straight  Muscle;  e  Outer  straight  Muscle, 
parallel  to  the  inner  straight  Muscle  on  the  other  side. 

The  motions  of  the  eye  are  effected  by  six  mus- 
cles, which  rise  from  the  bone  at  the  bottom  of 
the  socket,  and  are  fixed  to  the  eyeball.  Four  of 
these  are  placed  opposite  to  each  other,  and  are 
called  the  straight  muscles,  one  of  which  turns 
the  eye  upward  and  another  downward ;  another 
turns  it  toward  the  nose,  and  the  remaining  one  to- 
ward the  temple.  These  muscles  are  named  ac- 
cording to  their  actions.  "  The  upward  turning  of 
the  eye"  being  expressive  of  devotion,  the  upper 
muscle  is  sometimes  called  the  pious,  from  pro- 
ducing this  effect :  it  is  also  called  the  proud,  be- 
3 


26 


STRUCTURE   OF  THE  EYE. 


cause,  with  a  peculiar  disposition  of  the  muscles 
of  the  face,  it  is  partially  elevated  in  pride.  As  a 
downward  look  is  peculiar  to  modesty,  the  lower 
muscle  is  called  the  humble.  The  action  of  the 
muscle  which  draws  the  eye  outward,  causes  the 
sideward  looks  that  denote  contempt,  hence  its 
name  —  the  angry.  The  muscle  which  draws  the 
eye  inward  is  called  the  drinker,  from  directing 
the  pupil  to  the  bottom  of  the  cup  while  drinking. 

The  rolling  of  the  eye  is  caused  by  muscles 
which  are  placed  obliquely  to  its  axis.  The  lower 
oblique  commences  near  the  nose,  and  passes  un- 
der the  eye  to  the  outer  part  of  the  case,  where 
it  is  fixed.  The  upper  oblique  rises  from  the 
bottom  of  the  socket,  and  ends  in  a  cord,  which 
passes  through  a  ring,  and  then  turns  back 
to  be  fixed  to  the  outside  of  the  eyeball.  The 
passing  of  the  tendon  through  the  ring  resembles 
the  placing  of  a  rope  over  a  pulley  to  move  an  ob- 
ject in  the  required  direction,  while  an  additional 
contrivance  for  keeping  it  moist,  makes  it  move 
easily  like  machinery  which  is  oiled,  "  By  its 
six  muscles,"  observes  Dr.  Barclay,  "  the  eye, 
like  the  needle  of  the  mariner's  compass,  pointing 
to  the  pole,  preserves  the  same  relative  position 
with  regard  to  its  object,  whether  the  object  be  in 
motion  or  at  rest ;  and  hence  it  is,  that  instead  of 
the  eye  moving  in  its  socket  we  sometimes  see 


MUSCLES. 


27 


ihe  socket  moving  round  the  eye,  and  the  eye 
quite  still,  performing  its  functions." 

"  Each  of  these  muscles  is  provided  with  an  ad- 
versary. They  act  like  two  sawyers  in  a  pit,  by 
an  opposite  pull,  and  nothing  surely  can  more 
strongly  indicate  design  and  attention  to  an  end 
than  their  being  thus  stationed,  and  this  colloca- 
tion."* 


•  Paley. 


28 


STRUCTURE   OF  THE  EYE. 


When  a  magnifying  glass  is  fitted  into  a  hole  in 
the  window-shutter  of  a  dark  room,  und  the  light 
which  is  admitted  is  received  upon  a  sheet  of 
while  paper  held  at  a  certain  distance,  a  beautiful 
but  inverted  picture  of  everything  before  the  glass 
is  formed  on  the  paper.  The  representation  of 
the  scene  without  is  so  true  to  nature,  that  artists 
sometimes  avail  themselves  of  this  method  to 
make  a  correct  landscape.  The  things  essential 
to  this  experiment,  are:  1.  A  convex  glass,  to 
collect  and  concentrate  the  light  from  surrounding 
objects.  2.  An  opaque  or  semi-opaque  substance 
placed  at  the  focus  to  intercept  the  light  and  make 
the  image  visible ;  and  3.  A  covering  to  keep  the 
parts  in  situation.  The  eye  is  just  such  an  instru- 
ment, consisting  of  all  these  parts,  besides  others 
which  cannot  even  be  imitated,  and  it  is  construct- 
ed with  such  exquisite  workmanship,  that  even  the 
microscope  cannot  exhibit  the  minuteness  of  its 
structure. 


EYEBALL. 


29 


Fig.  19.-^SECTI0N  OF  THE  HUMAN  EYE,  NATURAL 
SIZE. 

a  Cornea  or  Window ;  b  Aqueous  Humour ;  c  Crystalline  Lens 
or  Principal  Magnifier ;  d  Vitreous  Humour :  e  Adjusters ;  g 
Optic  Nerve ;  h  Iris ;  i  Fold  of  Conjunctiva. 


Fig.  20.- PLAN. 

1  First  Magnifier  or  Cornea;  2  Aqueous  Humour;  3  Principal 
Magnifier;  4  Vitreous  Humour;  a  Outer  Case;  b  Second  Case; 
c  Retina. 

3* 


STRUCTURE  OF  THE  EYE. 


The  magnifiers  of  the  eye  are  :  1 .  the  transpa- 
rent window  through  which  we  see  the  coloured 
circle  surrounding  the  pupil ;  2.  a  quantity  of 
water  which  becomes  a  magnifier  by  the  shape  of 
the  skin  in  which  it  is  contained;  3.  the  crystal- 
line lens  which  is  the  principal  Magnifier ;  and  4. 
another  portion  of  water,  intersected  by  so  many 
skins  that  it  has  the  appearance  of  jelly. 

1.  The  first  magnifier,  which  is  the  window  of 
the  eye,  is  called  the  cornea,  and  resembles  a  small 
watch-glass,  fitted  to  the  outer  case  or  wliite  of  the 
eye,  like  the  glass  to  the  watch-case. 

2.  The  water  behind  the  window  is  of  the 
purest  transparency,  and  is  called  the  aqueous  hu- 
mour :  as  it  is  not  intersected  by  skins,  the  en- 
largement or  diminution  of  the  pupil  is  not  inter- 
rupted. 

3.  The  crystalline  lens  or  principal  magnifier, 
consists  of  a  series  of  coats  placed  under  each 
other  like  the  liths  of  an  onion,  and  these  again 
consist  of  fibres  which  are  more  compactly  ar- 
ranged as  they  approach  the  centre.  In  animals 
which  live  in  air,  this  magnifier  resembles  a  com- 
mon burning-glass,  for  the  most  part  convex  on 
both  sides,  but  occasionally  a  plano-convex.  In  fish 
it  is  either  round  like  a  pea,  (a  sphere,)  a  pea 
slightly  flattened,  (an  oblate  spheroid,)  or  a  pea 
elongated,  (a  prolate  spheroid.)    This  mafjnifier 


PLAN   OF  THE  COATS. 


31 


CASE 


REGULATORS 


MAGNIFIERS 


RETINA 
Four  Layers 


a  Cornea  or 

Window 
b  White  or 

Sclerotic 
c  Curtain 
Ad:  Muscles 


LINING 

Four  Layers  of^ 
which  the  2  and^ 
3  are  only  repre- 
sented. 


/  Ad :  Leaves 


( g  Principal 
I  Magnifier 


Vitreous  Hu- 
mour 


1  Vascular 
Membrane 


2  Fibres 


3  Globules 


^4  Jacob's  Coat 

r  1  Fold  of  Ja- 
cob's Coat 

2  Mondini's 

3  Venous 
1^4  Vascular 


CASE 


Sclerotic 


32 


STRUCTURE   OF  THE  EYE. 


is  contained  in  a  transparent  case,  the  edge  of 
which  passes  in  front  of  the  next  magnifier,  leav- 
ing an  unattached  part  between  them,  called  the 
canal  of  Petit,  for  the  purpose  of  allowing  it  to 
be  moved  backward  and  forward  for  proper  adjust- 
ment. With  all  the  light  of  modern  knowledge, 
no  artist  can  manufacture  a  magnifier  as  perfect 
as  that  possessed  by  the  meanest  animal.  Its 
structure  has  been  imitated  by  making  use  of  glass- 
es of  different  degrees  of  density,  with  the  result, 
that  optical  instruments  have  been  much  improved, 
although  the  imitation  is  far  from  perfect.  "  Could 
this  be  in  the  eye,  without  purpose,  that  suggested 
to  the  optician  the  only  means  of  attaining  that 
purpose  ?" 


Pig.  22.— FIBRES  OF  MAGNIFIER. 

Some  idea  of  the  extreme  minuteness  of  the 
workmanship  may  be  acquired  from  the  fact,  that 
Sir  David  Brewster  has  lately  ascertained,  that  the 
fibres  of  the  magnifier  of  the  cod  are  locked  to- 
gether by  a  kind  of  teeth  resembling  those  of 
rack-work.  He  found  the  number  of  teeth  in  each 
fibre  to  be  twelve  thousand  five  hundred.    As  the 


MAGNIFIERS. 


33 


magnifier  contains  about  five  million  fibres,  the 
number  of  these  minute  teeth  will  amount  to  sixty- 
two  billion,  five  hundred  million. 

The  material  of  which  the  principal  magnifier  is 
composed,  resembles  that  of  the  globules  of  the 
blood  deprived  of  colouring  matter.  As  more 
matter  is  required  for  the  formation  of  a  perfect 
sphere  than  for  a  part  of  one,  we  find  that  the  eyes 
of  animals  with  spherical  magnifiers  have  an  ap- 
paratus, apparently  for  the  purpose  of  supplying 
the  additional  demand.  At  the  back  of  the  eye, 
there  is  a  kind  of  gland  or  strainer  which  may  be 
easily  separated  into  two  portions  :  in  one,  the 
bloodvessels  divide  into  numerous  branches  termi- 
nating in  open  mouths,  close  to  the  mouths  of  the 
vessels  of  the  other.  The  prepared  material  given 
out  by  one  set  of  vessels  is  probably  taken  up  by 
the  other  and  carried  on  to  the  magnifier. 


Fig.  23.— STRAINER  OF  HALIBUT. 


34 


STRUCTURE  OP  THE  EYE. 


Like  other  portions  of  the  body,  the  principal 
magnifier  is  suppUedwith  bloodvessels  and  nerves. 
A  preparation  which  I  presented  to  Dr.  Knight  of 
Yale  College,  demonstrated  vessels  passing  from 
the  case  to  the  body  of  the  magnifier,  very  clearl}^ 

4.  About  three  fourths  of  the  hollow  globe  of 
the  eye  is  filled  with  water  contained  in  transpar- 
ent tells,  which  prevent  the  contents  of  the  eye 
from  running  out  when  wounded.  The  fluid  of 
of  the  opened  cells  only,  then  escapes,  and  the 
form  of  the  eye  is  preserved.  On  a  principle  some- 
what similar  ships  are  constructed,  divided  into 
various  compartments,  one  of  which  only,  fills 
when  a  plank  is  started,  while  the  buoyancy  of 
the  vessel  is  preserved  by  the  rest.  From  the  in- 
tersections of  the  membranes  forming  the  cells, 
the  fourth  magnifier  resembles  a  gelatinous  mass, 
which,  from  a  fancied  similarity  to  melted  glass, 
has  been  called  the  vitreous  humour.  In  the  struc- 
ture of  these  cells,  and  the  sources  whence  they 
derive  their  nourishment,  care  is  taken  to  avoid 
the  retina,  for  though  they  are  close  to  each  other, 
not  a  single  vessel  passes  from  the  one  to  the  oth- 
er throughout  the  whole  extent  of  the  latter,  with 
the  exception  of  a  solitary  vessel  that  proceeds 
from  the  entrance  of  the  optic  nerve,  a  spot  which 
is  blind,  and  which  does  not  therefore  require  the 
precautions  requisite  for  vision.    After  there  is  no 


MAGNIFIERS. 


35 


risk  of  interfering  with  the  retina,  the  membranes 
forming  the  cells  pass  backward  in  the  form  of  a 
star. 

The  magnifiers  just  described  collect  the  light 
in  such  a  manner  that  an  image  is  formed  on  the 
inner  layer  of  the  retina,  which  consists  of  ex- 
tremely minute  blood-vessels  branching  out  like 
the  veins  in  a  leaf,  after  the  soft  part  has  been  eaten 
away  by  insects.  From  the  network  appearance 
of  these  vessels,  the  whole  nervous  expansion  has 
received  the  name  of  the  retina.  As  the  light  pro- 
ceeding from  a  magic  lantern  or  through  the  mag 
nifier  into  a  dark  room,  would  not  form  an  image 
unless  there  was  a  sheet  of  paper  or  other  opaque 
or  semi-opaque  substance  to  intercept  it,  so  the 
light  passing  through  the  magnifiers  of  the  eye 
must  fall  on  a  similar  substance  before  it  can  form 
an  image.  This  network,  then,  may  be  called  the 
screen  on  which  the  image  is  received.* 

An  image  of  the  coarser  vessels  of  the  network 
resembHng  a  withered  tree  exhibited  on  a  screen 
will  soon  appear,  if  when  the  eye  is  directed  stead- 
ily forward,  we  move  a  lighted  candle  np  and  down 
on  one  side  of  the  line  of  vision.  The  vessels  ap- 
pear much  magnified  on  account  of  the  proportion 
of  the  nervous  expansion  they  cover  when  compar- 


♦  For  the  r.ppearance  of  the  vessels,  globules,  &c.,  see  Fig.  21. 


36 


STRUCTURE  OF  THE  EYE. 


ed  with  that  of  an  ordinary  image.  This  representa- 
tion nnay  be  owing  to  pressure  on  the  vessels  by  a 
swelling  of  the  nervous  fibres,  produced  by  a  vio- 
lent and  irregular  play  of  light  upon  them  ;  or  it 
may  be  the  effect  of  violent  exercise  of  the  colour- 
ed circle  round  the  pupil,  and  consequent  accu- 
mulation in  the  neighbouring  vessels. 

The  strings  of  the  expanded  nerve  are  spread 
over  the  network,  and  on  these  again  there  is  a 
layer  of  minute  globules  retained  in  their  place  by 
a  fine  skin  which  is  called  the  coat  of  Jacob. 

The  rays  of  light  being  collected  by  the  mag- 
nifiers, and  intercepted  by  the  network  or  screen, 
cause  a  vibration  of  the  nervous  fibres  on  the 
globules  behind  them,  and  these  vibrations  be- 
ing communicated  along  the  nerve,  which  after 
joining  its  fellow  of  the  opposite  side  proceeds  to 
the  brain,  vision  is  the  consequence. 

That  both  surfaces  of  the  retina  are  not  equally 
sensible  to  light  may  be  illustrated  by  an  experi- 
ment related  by  Sir  Charles  Bell  :  "  Close  the 
eyelids  and  cover  them  with  a  piece  of  black  cloth 
or  paper  that  has  a  small  hole  in  it,  and  place  this 
hole,  not  opposite  to  the  pupil,  but  to  the  white  of 
the  eye  ;  direct  a  beam  of  light  upon  the  hole  :  a 
person  will  see  this  light  in  its  true  direction."  In 
this  experiment  the  light  falls  upon  two  parts  of 
the  retina  ;  the  same  or  a  greater  impulse  is  given 


RETINA, 


37 


to  the  fibres  first  struck,  but  we  see  only  one  circle 
of  light.  When  the  hght  passes  through  the  reti- 
na in  the  first  instance,  it  forces  the  fibres  against 
the  vascular  membrane  without  producing  an  im- 
pression, but  when  it  strikes  upon  their  concave 
surface,  and  impels  them  against  the  globules,  the 
light  is  seen  in  the  true  direction. 


Fig.  24.— FIBRES  OF  RETINA. 
a.  Entrance  of  Optic  Nerve. 

There  is  an  opening  with  a  yellow  margin  in 
the  centre  of  the  retina  in  man,  round  which  the 
ends  of  some  of  the  nervous  fibres  meet.  The 
probable  use  of  this  arrangement  is  to  enable  him 
to  see  very  minute  objects,  for  as  sensation  is  most 
acute  at  the  extremities  of  nerves,  the  impression 
from  a  minute  object  received  on  the  ends  of  the 
fibres  thus  collected,  will  be  more  powerful  than 
elsewhere,  just  as  a  stroke  on  the  end  of  a  wire 
causes  a  greater  vibration  than  on  the  middle.  In 
order  to  give  nourishment  to  these  fibres,  as  well 
4 


38 


STRUCTURE   OF  THE  EYE. 


as  to  form  the  meshes  of  the  network  above  de- 
scribed, a  bloodvessel  enters  with  the  optic  nerve, 
and  in  its  course  it  avoids,  by  passing  round  it, 
this  yellow  spot,  which  is  the  most  sensible  part 
of  the  e5^e.  The  care  which  has  been  taken  to 
keep  the  trunk  at  a  distance  is  also  extended  to 
the  branches,  for  no  coarse  vessel  is  permitted  to 
approach  this  delicate  structure  and  thus  interfere 
with  vision. 


Fig.  25. -BLOODVESSELS  OF  RETINA. 

We  sometimes  see  a  portion  of  the  network,  of 
the  fibres,  or  of  the  globules,  floating  before  the 
eyes  when  they  have  been  for  some  time  exposed 
to  a  very  bright  light,  as  after  riding,  when  the 
ground  is  covered  with  snow.  The  cause  of  the 
apparent  motion  seems  to  be  this :  As  other  nerves 
become  erected  so  the  nervous  fibres  or  strings 
become  erect  or  tuned  like  a  musical  instrument, 
in  order  to  be  placed  in  a  proper  condition  for  dis- 
tinct vision.  Should  there  be  any  unusual  fulness 
of  the  reticulated  vessels  or  any  displacement  or 
deficient  energy  of  the  strings  or  of  the  globules  — 


RETINA. 


39 


the  strings  will  not  be  free  to  vibrate  by  the  light, 
but  will  convey  false  impressions,  when,  during 
iheir  erection  or  tension,  they  come  in  contact  with 
bloodvessels,  nervous  fibres,  or  globules,  out  of 
their  natural  order. 

As  the  image  of  a  grain  of  a  sand,  or  a  still 
smaller  object  must  be  very  minute,  when  repre- 
sented on  the  retina,  a  very  slight  tension  of  the 
strings  would  make  the  bodies,  to  which  we  refer 
seem  to  pass  over  a  great  space. 

One  of  the  greatest  discoveries  in  modern  phys- 
iology, is  that  of  Sir  Charles  Bell,  who  ascertain- 
ed that  there  are  two  sets  of  nervous  fibres  —  one 
for  sensation  and  one  for  motion.  By  the  one  the 
impressions  from  external  objects  are  communica- 
ted to  the  brain,  while  the  mandates  of  the  will 
are  conveyed  from  the  brain  by  the  oiher.  Each 
of  the  organs  of  the  senses  is  supplied  with  differ- 
ent nervous  fibres,  one  set  for  sensation  and  anoth- 
er for  adjusting  the  apparatus  by  means  of  which 
the  sensation  is  effected.  Magendie,  an  eminent 
French  physiologist,  found  that  when  the  adjusting 
nerve  of  the  eye  was  cut  across,  the  animal  became 
tis  blind  as  when  the  optic  nerve  itself  was  divi- 
ded. 

That  objects  are  inverted  on  the  retina  may  be 
easily  shown  by  cutting  off  the  posterior  cover- 
ings of  the  eye  of  one  of  tlie  lower  animals.  The 


40 


STRUCTURE  OF  THE  EYE. 


demonstration  may  also  be  made  by  an  apparatus 
which  I  have  invented. 


c  ha 
Fig.  26. 


a  is  a  square  box  on  the  side  of  which  is  a  trans- 
parent painting ;  6,  is  a  pyramidal  box  to  the  small 
end  of  which  is  adapted  an  artificial  eye  c.  When 
a  light  is  placed  behind  the  transparency,  and 
the  boxes  closely  applied,  a  beautiful  inverted 
representation  of  the  transparency  may  be  seen  on 
the  artificial  retina,  when  the  room  is  darkened. 

The  reason  why  we  see  objects  erect,  although 
the  image  is  inverted  is  a  subject  upon  which  a 
great  deal  has  been  written.  The  explanation 
is  simply  this  —  the  fibres  of  the  retina  when 
struck  on  one  side  of  the  nervous  expansion,  con- 
vey to  the  brain  an  opposite  impression. 

If  I  shut  my  eyes  and  press  the  retina  of  one 
of  them  on  the  outside,  the  circle  of  light  which 
is  thus  produced  will  appear  as  if  proceeding  from 
the  inside.  If  I  press  above,  the  circle  will  seem 
below,  and  if  I  carry  the  finger  completely  round 


RETINA. 


41 


the  eye,  the  light  will  always  be  opposite  to  the 
finger.  If  with  this  disposition  of  the  nerve,  the 
picture  were  not  inverted,  everything  would  ap- 
pear upside  down. 

From  the  inverted  picture  on  the  retina,  and 
from  the  facts  that  children  miss  the  object  at  which 
they  grasp,  and  that  a  person  who  had  been  born 
blind,  afler  restoration  to  sight  by  an  operation, 
could  not  at  first  see  correctl}^,  it  was  inferred  that 
everything  really  appears  upside  down,  but  that 
the  error  Vv^as  corrected  by  the  sense  of  touch. 
Although  it  is  obvious  that  the  eyes  of  children 
are  not  perfect  for  a  considerable  time  after  birth, 
and  that  eyes  which  have  been  couched  are  de- 
prived of  the  use  of  the  principal  magnifier,  it  is 
remarkable  that  the  opinion  that  we  do  not  see 
correctly  unless  we  learn  to  do  so  by  experience, 
is  still  maintained  by  most  authors  on  the  subject. 
A  chicken,  as  soon  as  it  is  hatched,  without  any 
education  of  the  sense  of  sight,  can  pick  up  a  seed, 
with  unerring  certainty,  and  the  sparrow  and  the 
bee  fly  in  correct  directions  at  the  very  first  at- 
tempt. Although  we  are  convinced  by  the  sense 
of  touch,  that  an  oar  may  be  straight,  yet  when 
partially  immersed  in  water,  it  will  seem  crooked  ; 
and  when  we  look  at  a  long  row  of  trees  of  equal 
height,  the  one  which  is  most  distant  will  appear 
the  shortest,  notwithstanding  our  experience  to  the 
4* 


42 


STRUCTURE  OF  THE  EYE, 


contrary.  The  Divine  Architect  has  formed  every 
sense  perfect  in  itself,  and  independent  of  any  other. 

The  extreme  minuteness  of  the  inverted  pic- 
ture is  far  beyond  our  comprehension.  "  A  whole 
printed  sheet  of  newspaper,"  says  Dr.  Arnott, 
"  may  be  represented  on  the  retina  on  less  sur- 
face than  that  of  a  finger-nail,  and  yet  not  only 
shall  every  word  and  letter  be  separately  perceiva- 
ble, but  even  any  imperfection  of  a  single  letter ; 
or,  more  wonderful  still,  when  at  night  an  eye  is 
turned  up  to  the  blue  vault  of  heaven,  there  is 
portrayed  on  the  little  concave  of  the  retina,  the 
boundless  concave  of  the  sky,  with  every  object 
in  its  just  proportions.  There  a  moon  in  beauti- 
ful miniature  may  be  sailing  among  her  white- 
edged  clouds,  and  surrounded  by  a  thousand  twink- 
ling stars,  so  that  to  an  animalcule,  supposed  to  be 
within  and  near  the  pupil,  the  retina  might  appear 
another  starry  firmament  with  all  its  glory.  If 
the  images  in  the  human  eye  be  thus  minute,  what 
must  they  be  in  the  little  eye  of  a  canary  bird,  or 
of  another  animal  smaller  still !" 

The  optic  nerve  does  not  enter  the  eye  at  the 
centre,  but  at  a  little  toward  the  inner  side.  (Fig. 
24,  o,  and  Fig.  25.)  Had  it  entered  at  the  focus 
of  the  lenses,  vision  would  have  been  indistinct,  for 
it  is  insensible  at  the  part  where  it  expands  to  form 
the  retina.    In  order  to  be  convinced  of  this,  place 


RETINA. 


43 


two  wafers  on  a  wall  about  a  foot  apart,  and  with 
the  right  eye  directed  to  the  one  on  the  left,  shut 
the  other.  When  the  observer  is  near,  both  ob- 
jects are  seen,  but  by  withdrawing  three  or  four 
feet,  one  of  them  becomes  invisible,  because  the 
light  from  it  strikes  upon  the  entrance  of  the 
nerve,  a  spot  where  the  fibres  are  bound  together, 
and  where  no  vibration  can  be  effected ;  just  as 
the  stroke  on  the  end  of  a  bunch  of  wires  would 
not  cause  them  to  vibrate,  if  it  fall  in  the  direction 
of  the  wires.  By  withdrawing  further,  the  light 
from  the  wafer  falls  on  another  part  of  the  retina, 
and  the  object  again  appears.* 

As  the  nerve  where  it  enters  each  eye  is  abso- 
lutely blind,  we  should  expect  to  see  two  dark 
spots  on  every  landscape  :  but  as  has  been  ob- 
served by  Sir  David  Brewster,  "  the  Divine  Artifi- 
cer has  not  left  his  work  thus  imperfect.  Though 
the  base  of  the  optic  nerve  is  insensible  to  hght 
that  falls  directly  upon  it,  yet  it  has  been  made 
susceptible  of  receiving  luminous  impressions  from 
the  parts  which  surround  it,  and  the  consequence 
of  this  is,  that  when  the  wafer  disappears,  the  spot 
which  it  occupied,  in  place  of  being  black,  has 
always  the  same  colour  as  the  ground  upon  which 
the  wafer  is  laid." 


*  Mariotte, 


44 


STRUCTURE   OF  THE  EYE. 


The  vibrations  of  the  strings  of  the  retina  con- 
tinue for  about  the  sixth  of  a  second.  When  a 
stick  burning  at  one  end  is  turned  rapidly  round, 
the  whole  of  the  circle  through  which  it  passes 
becomes  lunainous.  At  every  change  of  place,  the 
vibrations  caused  by  the  light  from  the  burning- 
wood  are  renewed  before  the  previous  vibration 
ceases,  and  the  appearance  of  a  continued  circle  is 
thus  produced.  If  the  impression  of  light  did  not 
remain  for  some  time,  the  sight  would  not  be 
steady,  for  objects  would  disappear  while  winking. 

The  duration  of  the  impressions  has  been  studied 
in  the  construction  of  various  toys.  The  wonder- 
turner  of  Dr.  Paris  consists  of  a  card,  on  one  side 
of  which  is  painted  a  part  of  the  objects  intended 
to  be  represented,  and  on  the  other  side  the  re- 
maining part.  When  the  card  is  turned  rapidly 
round  by  means  of  twisted  strings,  at  each  side 
the  objects  appear  together.  For  example,  if  a 
man  be  painted  on  one  side  of  the  card,  and  a 
horse  upon  the  other ;  or  if  a  bird  be  painted  on 
one  side,  and  on  the  other  a  cage  :  when  the  card 
is  turned  round  the  man  will  appear  on  horseback, 
or  the  bird  will  seem  imprisoned  in  the  cage. 

The  phantasmascope  consists  of  a  circular  card, 
on  which  are  painted  figures  gradually  changing 
from  one  attitude  to  another.  The  margin  is 
painted  of  a  dark  colour,  and  perforated  with  nu- 


RETINA. 


45 


merous  apertures.  If,  when  the  card  is  turned 
round,  we  look  through  the  apertures  before  a  mir- 
ror, the  figures  will  appear  in  active  motion,  be- 
cause the  impression  from  those  at  one  extreme  of 
attitude  is  gradually  lost  before  the  card  is  turned 
round  to  the  other  extreme.  The  dark  space  be- 
tween the  apertures  removes  the  figure  for  a  mo- 
ment, but  the  impression  remains  ;  another  figure 
is  then  presented,  the  attitude  of  which  is  so 
slightly  changed  that  the  eye  cannot  perceive  the 
difference  :  the  figures  in  the  other  attitudes  are 
successively  removed,  a  new  one  being  presented 
before  the  previous  impression  ceases.  The  eye 
becomes  deceived  by  the  mixture  of  impressions, 
and  the  appearance  of  objects  in  motion  is  pro- 
duced. In  this  way  a  smith  will  seem  to  strike 
his  anvil,  a  harlequin  to  dance,  or  a  horse  to  run  a 
race. 

The  anorthoscope  is  made  by  fixing  to  an  axis 
two  circular  papers — the  one  dark  and  opaque,  and 
perforated  by  four  slits  ;  and  the  other  having  a 
transparent  distorted  figure  painted  on  it.  When 
placed  before  a  candle  and  revolved  in  opposite 
directions,  the  distorted  figures  are  reduced  to  cor- 
rect proportions,  and  their  number  is  increased  ac- 
cording to  the  perforations.  The  impressions  from 
the  different  portions  of  the  distorted  objects  come 
so  rapidly  before  the  perforation  that  they  are  all 


46 


STRUCTURE   OF  THE  EYE. 


blended  together,  and  the  size  of  the  figure  is  reg- 
ulated by  the  size  of  the  aperture  in  the  opaque 
paper. 

The  eyes  of  those  fishes  which  are  furnished 
with  a  hydrostatic  apparatus  for  forcing  out  the 
eye,  are  protected  by  a  thick  layer  of  fat  or  jelly, 
of  all  animal  substances  free  from  air,  the  worst 
conductors  of  heat.  The  animal  heat  is  thus  pre- 
served, and  the  retina  kept  in  a  proper  condition 
for  receiving  impressions.    (See  Figs.  36,  37,  41.) 

Immediately  behind  the  window  of  the  eye, 
and  immersed  in  the  water  which  lies  before  the 
principal  magnifier,  is  the  circle  called  the  iris, 
which  gives  colour  to  the  eye,  and  which,  by  be- 
coming larger  or  smaller,  regulates  the  admission 
of  light.  The  pupil  is  merely  a  hole  in  the  centre 
of  this  circle,  and  it  is  dark,  owing  to  the  lining  of 
the  eye. 

The  retina  is  unable  to  bear  a  sudden  or  too 
strong  light,  for  we  know  that  persons  have  be- 
come blind  by  sudden  exposure  to  the  direct  rays 
of  the  sun.  It  is  said  of  Dionysius,  the  tyrant  of 
Syracuse,  that  he  used  to  bring  his  prisoners  from 
the  dark  dungeons  in  which  they  were  confined, 
into  a  white,  well  lighted  room,  and  that  the  sud- 
den transition  from  darkness  to  light,  immediately 
blinded  the  unfortunate  victims  of  his  cruelty. 
Northern  travellers  are  obliged  to  protect  their 


IRIS. 


47 


eyes,  by  wearing  spectacles  which  admit  only  a 
very  small  portion  of  light  through  a  hole  in  the 
centre.  To  guard  the  retina,  therefore,  and  to 
regulate  the  admission  of  light,  the  iris  is  placed 
as  a  watchful  sentinel.  When  the  light  is  strong, 
the  diameter  of  the  pupil  is  contracted,  to  prevent 
the  admission  of  too  great  a  quantity,  and  when 
feeble,  it  is  expanded,  to  admit  what  niay  be  ne- 
cessary. 

This  coloured  circle  round  the  pupil,  or  curtain 
of  the  eye,  is  full  of  blood-vessels,  the  fluid  in 
which  is  controlled  by  a  series  of  muscular  fibres, 
placed  at  the  circumference.  By  the  contraction 
of  these  fibres,  the  blood  is  collected  in  the  ves- 
sels, which  thus  become  lengthened,  and  the  size 
of  the  pupil  diminished,  while  it  is  increased  by 
the  relaxation  of  the  fibres,  permitting  the  passage 
of  the  blood,  and  allowing  the  vessels  to  become 
smaller 

The  e3^es  of  those  animals  that  obtain  their  food 
at  night,  are  furnished  with  a  bright  reflector, 
which  prevents  the  total  loss  of  light  by  absorp- 
tion, and  by  reflecting  it  again,  produces  sufficient 
agitation  of  the  fibres  of  the  retina,  when  the  light 
is  feeble.  In  a  strong  light,  a  retina,  accompanied 
by  a  reflector,  would  be  more  violently  exercised 
than  without  such  an  accompaniment.  Accord- 
ingly we  find,  that  to  exclude  unnecessary  light, 


48 


STRUCTURE  OF  THE  EYE. 


the  iris  in  nocturnal  animals  is  capable  of  great 
contraction.  Of  the  fishes  I  have  examined,  the 
skate  is  the  only  one  furnished  with  a  reflector : 
to  protect  the  retina,  which  is  in  consequence  un- 
usually sensible,  there  is  a  curtain  at  the  upper 
part  of  the  pupil,  having  the  appearance  of  a 
hand,  the  fingers  of  which  may  be  so  expanded, 
that  the  light  may  be  altogether  excluded  or  admit- 
ted, only  through  very  small  chinks. 


Pig.  25.-EYE  OF  SKATE. 

During  sleep  the  pupil  is  contracted  and  turned 
upward,  and  the  eyelid  drawn  down;  three  pro- 
visions are  thus  adopted  to  exclude  the  light,  and 
allow  the  nervous  fibres  repose.* 

The  form  of  the  pupil  varies  in  different  an- 
imals. In  some  it  is  circular ;  in  the  sheep  and 
the  ox,  it  is  oblong  and  horizontal,  that  they  may 


*  Bell. 


IRIS. 


49 


see  a  large  portion  of  the  meadows  on  which  they 
graze  ;  while  in  the  cat  it  is  perpendicular,  that  it 
may  see  better  up  and  down.* 


*  Porterfield. 


5 


60 


STRUCTURE  OP  THE  EYE. 


THE  APPARATUS  BY  WHICH  THE  EYE  IS  ADJUSTED 
TO  DISTANCES  IN  DIFFERENT  ANIMALS. 

As  the  magnifier  or  screen  in  the  dark  chamber, 
requires  to  be  shifted  according  to  the  distance  of 
the  object,  so  a  change  must  be  effected  in  the  eye 
when  looking  at  near  or  at  distant  objects,  in  or- 
der that  a  perfect  image  may  be  formed  on  the  re- 
tina. In  the  eye,  as  there  is  no  provision  for  mo- 
ving the  screen,  the  effect  is  produced  by  chan- 
ging the  position  of  the  magnifier. 

In  the  shifting  of  an  ordinary  magnifier,  we 
must  be  careful  to  keep  every  part  of  the  edge  at 


Figs.  30,  31,  32,  33,  34,  35.— PLANS. 


ADJUSTMENT. 


51 


an  equal  distance  from  the  space  it  formerly  oc- 
cupied, or  it  will  be  turned  away  from  the  object ; 
we  cannot,  therefore,  adjust  it  by  traction  at  a  sin- 
gle point,  (Fig.  31.)  What  happens  in  this  re- 
spect to  a  magnifier,  which  is  less  than  a  sphere, 
will  also  happen  to  one  which  is  greater,  (Fig.  30,) 
but  with  a  perfect  sphere,  it  is  immaterial  whether 
the  traction  be  made  at  a  single  point,  or  at  a  num- 
ber, (Fig.  32.)  This  may  be  illustrated  by  the 
preceding  sketches. 

If  we  attach  a  thread  to  one  point  of  a  magnifier, 
which  is  a  perfect  sphere,  and  draw  it  forward,  the 
latter  will  be  turned  on  its  axis,  and  as  the  diame- 
ters of  a  sphere  are  always  the  same,  there  will  be 
no  change  in  the  direction  of  the  hght  which  pass- 
es through  it,  (Fig.  32  ;)  but  if  the  magnifier  be 
less  than  a  sphere,  as  in  man,  or  larger  than  a 
sphere,  as  in  the  cuttle-fish,  and  drawn  in  the  same 
manner,  unequal  diameters  will  be  presented  for 
the  passage  of  light,  and  a  distinct  image  will  not 
be  formed,  (Fig.  30  and  31.)  With  that  economy 
which  exists  in  all  the  works  of  nature,  we  find 
only  a  single  instrument  for  adjusting  the  magni- 
fier, when  one  alone  will  answer  the  purpose,  but 
without  parsimony  there  are  as  man}^  as  can  be 
packed  together,  whenever  their  services  are  re- 
quired. 

In  a  large  perch  found  on  the  American  coast, 


52 


STRUCTURE  OP  THE  EYE. 


Figs.  36,  37,  38,  39,  40.— STRIPED  BASS. 

called  the  striped  bass,  there  is  a  triangular  mus- 
cle attached  to  the  sheath  of  a  nerve  which  enters  at 
the  back  of  the  eye  and  runs  along  a  division  at  the 
the  lower  part  of  the  retina.  One  of  the  angles  of 
the  muscle  is  fixed  to  the  magnifier,  and  anothci 
after  passing  through  a  loop  at  the  back  of  the 
coloured  circle  round  the  pupil,  is  inserted  into  the 
membranes  of  the  vitreous  humour.  When  the 
muscle  contracts,  the  magnifier  is  drawn  forward 


ADJUSTMENT. 


53 


and  adjusted  to  near  objects,  while  it  is  drawn 
back  again  or  regulated  to  distant  objects,  by  the 
elasticity  or  spring  of  the  membranes  of  the  vitre- 
ous humour. 


Figs.  41,  42.— EYE  OF  HALIBUT. 
a.  Adjuster,  b  Gelatinous  Fluid,  c  Choroid  Gland  or  Strainer. 


In  the  halibut,  the  muscle  which  is  of  a  differ- 
ent shape,  and  larger  than  in  striped  bass,  does  not 
pass  through  a  loop,  but  lies  diagonally  across  the 


Fig.  43.— RETINA  OF  HALIBUT 

a.  Division  in  the  Rctina- 
5.h= 


STRUCTURE  OF  THE  EYE. 


eye,  having  one  attachment  to  the  magnifier,  and 
another  to  the  colom'ed  circle  round  the  pupil 
The  magnifier  is  drawn  forward  when  the  muscle 
acts  and  when  the  power  is  removed,  it  is  drawn 
back  again,  by  the  elasticity  of  the  membranes  of 
the  vitreous  humour. 

To  aid  the  retraction,  the  membranes  of  the  vit- 
reous humour  pass  through  a  slit  in  the  retina,  and 
are  fastened  to  the  strong  outer  coats,  (Fig.  43.) 

Though  the  single  adjuster  varies  in  shape  and 
size,  it  may  always  be  found  whenever  the  mag- 
nifier is  a  sphere.  When  the  magnifier  is  less 
than  a  sphere,  as  in  animals  which  live  in  air,  and 
some  inhabitants  of  the  water  as  the  shark,  the  por- 
poise, &c.,  and  whenever  it  is  greater,  as  in  the 
cuttle-fish,  there  are  numerous  adjusters  closely 
arranged  round  the  circumference. 


Fig.  44.-CILfARY  PROCESS,  OR  ONE  OF  THE  ADJUST- 
ING LEAVES  OF  THE  OX,  MAGNIFIED. 

Immediately  behind  tl)e  coloured  circle  round 


ADJUSTMENT. 


55 


the  pupil,  and  attached  to  the  extended  margin 
of  the  case  of  the  magnifier,  there  are  about  sev- 
enty or  eighty  vascular  membranes,  resembling 
leaves,  and  called  the  ciliary  processes,  which  are 
placed  round  the  magnifier,  like  the  petals  of  a  flow- 
er. When  the  magnifiers  are  separated  from  the 
cases,  these  leaves  adhere  so  firmly  that  the  separ- 
ation can  only  be  effected  by  tearing  them  asun- 


Fig.  45.-EYE  OF  SHEEP. 
a  Principal  Magnifier,  6  Portion  of  Adjusting  Leaves,  c  Impres- 
sions from  Upper  Adjusting  Muscle,  d  Vitreous  Humour. 


Fig.  46.— FRONT  VIEW  OF  ADJUSTING  MUSCLES  IN 
MAN. 


56 


STRUCTURE  OF  THE  EYE. 


Fig.  47.— FRONT  VIEW  OF  ADJUSTING  MUSCLES 
IN  OX 

a  Upper,  h  Lower  Muscle,  c  Magnifier,  d  Bloodvessels. 


Fig.  49.  -HUMAN  CILIARY  VEINS. 
a  Natural  Size,  h  Iris,  c  Adjusting  Leaves,'  r?,  e  The  veins 
returning  from  them. 


ADJUSTMENT. 


67 


der.  The  portion  which  remains  forms  a  kind  of 
circle,  (Fig.  45,  b.) 

At  the  roots  of  the  adjusting  leaves  there  are 
muscles  with  radiating  fibres,  which  are  covered 
by  a  white  ligamentous  substance,  and  a  thin  dark 
skin  which  lines  the  eye,  and  keeps  them  from  be- 
ing readily  observed.  The  vessels  which  supply 
the  adjusting  leaves,  pass  at  the  junction  of  the 
muscle  —  a  spot  where  they  are  not  affected  by 
pressure,  (Fig.  47,  d,)  while  the  returning  veins 
are  so  situated  that  the  progress  of  the  blood  as 
it  passes  through  them,  may  be  arrested,  (Fig. 
49,  c,  d.) 

"When  the  muscles  contract,  an  additional  quan- 
tity of  blood,  is  collected  in  the  adjusting  leaves, 
which  being  consequently  elongated,  draw  forward 
the  margin  of  the  case  to  which  they  are  attached, 
and  of  course  the  magnifier  which  it  envelopes,  in 
this  manner  adjusting  the  organ  to  the  vision  of 
near  objects,  while  it  is  drawn  back  or  adjusted  to 
those  which  are  distant,  by  the  elasticity  or  spring 
of  the  membranes  of  the  vitreous  humour  which 
radiate  from  the  magnifier. 

The  plan  of  the  adjustment  of  the  eye,  may  be 
familiarly  illustrated  in  the  following  manner  : — 

Cut  out  of  an  apple  a  piece  resembling  a  mag- 
nifying-glass,  which  may  represent  the  principal 
magnifier  the  remainder  of  the  apple  representing 


68 


STRUCTURE  OF  THE  EYE. 


the  vitreous  humour.  Cover  this  magnifier  with 
cloth  to  represent  the  case,  and 
put  a  ruffle  round  tlie  edge  for  its 
margin.  For  ciliary  processes, 
fix  to  the  ruffle,  by  their  tops  and 
sides,  pieces  of  clolh  or  paper,  resembling  leaves, 
with  their  sides  pressed  together,  and  folded  down 
at  the  top,  as  in  the  cut.  Attach  ihe  magnifier  to 
its  socket  by  a  few  threads  of  India-rubber,  and 
fix  the  edge  of  the  ruffle  to  the  vitreous  humour  at 
the  dotted  line.  Unfold  the  leaves,  and  the  ruffle 
with  the  lens  attached  to  it  will  be  drawn  for- 
ward; close  them  again  and  it  will  be  drawn 
back. 


Fig.  51.— BLOOD-VESSELS  OF  THE  EYE. 

a  Carotid  artery  :  b  Ophthalmic  artery  ;  c  Optic  nerve. 


ADJUSTMENT. 


59 


Fig.  52.- EYE  OF  LYNX. 
a  Ciliary  Processes  or  Adjusters. 

To  prevent  the  adjustment  being  disturbed  by 
the  beats  of  the  heart,  there  are  bendings  in  the 
blood-vessel  from  which  the  eye  is  supplied  ;  and 
after  the  appropriate  stream  has  been  given  off, 
the  force  of  the  current  is  still  further  diminished, 
by  numerous  divisions  and  additional  bendings. 
In  the  cat  and  the  dog— animals  which  watch  their 
prey  for  a  long  time — the  division,  subdivision, 
and  reunion  of  the  vessels  before  entering  the  eye, 
are  very  remarkable.  The  beats  of  the  heart  are 
not  felt  after  these  divisions,  and  the  eye  does  not 
lose  its  object. 

The  lynx  has  long  been  considered  the  most 
quicksighted  of  animals.  From  the  size  of  the 
adjusters,  it  should  possess  a  great  range  of  vision. 

Bv  the  eighty  processes  arranged  round  its  cir- 
cumference, the  crystalline  lens  is  drawn  back-. 


60 


STRUCTURE  OP  THE  EYE. 


ward  and  forward,  and  even  a  little  obliquely,  like 
a  magnifying-glass  with  eighty  strings  at  its  edge, 
pulling  it  and  adjusting  it  to  fit  the  distance  and 
the  direction  of  the  object. 

The  experinnents  of  Audubon  show  that  the 
turkey-buzzard  does  not  discover  carrion  by  smell ; 
yet,  though  very  remote,  it  will  find  its  prey  when 
a  carcass  is  exposed.  The  observations  of  others 
on  birds,  prove  that  they  see  at  a  great  distance. 
Their  adjusting  apparatus  is  of  great  breadth,  and 
made  stiff*  by  plates  of  bone  wedged  into  each 
other.  A  strong  fixture  is  thus  afforded  the  ad- 
justers, when  they  require  unusual  exertion,  and 
their  effect  may  be  increased  by  the  marsupium. 


Fig.  53.— EYE  OF  EAGLE. 

a  Marsupium. 

In  birds,  a  skin  called  the  marsupium^  which 
resembles  a  fan,  arises  at  the  back  of  the  eye, 


ADJUSTMENT 


61 


where  it  passes  through  a  division  in  the  retina, 
and  is  inserted  into  the  vitreous  humour,  not  far 
from  the  foremost  or  inner  edge  of  the  magnifier, 
out  of  the  w^ay  of  direct  vision.  The  eyes  of 
birds  are  directed  to  each  side  to  afford  the  wide 
field  of  vision  which  their  necessities  require ;  yet 
when  this  membrane  contracts,  it  will  alter  the 
position  of  the  magnifier,  by  drawing  back  the 
inner  edge,  and  thus  enable  the  bird  to  look  straight 
forward.  From  numerous  dissections,  I  believe  it 
to  be  established,  that  the  marsupium  becomes 
smaller,  in  proportion  as  the  direction  of  the  eyes 
become  parallel.  By  drawing  back  the  magnifier, 
this  membrane  will  assist  in  accommodating  the 
eyes  of  birds  to  distances  ;  but  as  the  direction  of 
the  light  would  be  then  changed,  the  organs  can- 
not be  regularly  adjusted  in  this  manner. 


a 


Fig.  54  .-LINING, 
a  Choroid  or  lining ;  6  Sclerotica ;  c  Optic  Nerve  : 

Attached  to  the  adjusting  apparatus,  and  envel- 
oping the  semi-transparent  retina,  there  is  a  vas- 


STRUCTURE  OF  THE  EYE. 


cular  membrane  called  the  choroid,  which  pre- 
pares the  globules  of  dark  paint  that  line  the  eye, 
for  the  purpose  of  absorbing  unnecessary  light,  and 
making  a  more  distinct  image.  The  vessels  of 
which  it  consists,  spread  out  like  the  branches  of 
a  weeping  willow,  and  in  their  passage  through 
the  strong  outer  case,  or  white  of  the  eye,  they 
are  kept  at  a  distance  from  the  aperture  which  ad- 
mits the  optic  nerve.  If  they  passed  through  the 
same  aperture,  partial  or  total  blindness  would  oc- 
cur at  every  violent  exertion,  from  the  pressure  of 
the  blood  against  the  nervous  fibres. 

The  anterior  fifth  of  the  outer  case,  is  formed 
by  the  cornea  or  window  of  the  eye,  which  has 
been  already  described  ;  while  the  remaining  four 
fifths  is  formed  by  a  strong  tough  membrane, 
which,  by  its  firmness,  keeps  the  parts  in  situation, 
and  receives  the  attachments  of  all  the  muscles 
that  give  motion  to  the  eyeball.  This  strong  cov- 
ering constitutes  the  white  of  the  eye,  which  is 
called  the  sclerotica,  from  its  hardness. 


CONCLUSION. 


63 


In  the  eye,  we  find  an  instrument  made  perfect 
for  ihe  purpose,  with  the  utmost  economy  of  ma- 
terial. As  tears  would  be  of  no  use  to  the  inhab- 
itants of  the  deep,  no  organs  are  provided  for  them ; 
but  where  they  are  required,  there  is  a  gland  for 
preparing  them,  and  a  channel  for  carrying  them 
away.  When  the  crystalline  lens  may  be  adjusted 
by  the  pulling  of  a  single  string,  a  single  string  is 
all  that  we  find  ;  but  when  action  at  only  one  point 
would  alter  the  direction  of  the  light,  the  requisite 
strings  are  liberally  supplied.  According  to  the 
danger  to  which  the  organ  is  exposed  there  are 
suitable  provisions  for  defence,  but  in  no  instance 
are  they  found  where  they  are  not  absolutely  re- 
quired. Wisdom,  power,  and  goodness,  are  mani- 
fest in  the  whole  structure.  The  bountiful  Creator 
has  provided  an  organ  suited  to  the  wants  of  His 
creatures,  and  with  consummate  knowledge  He 
has  varied  it  according  to  the  demand. 

When  the  most  exquisite  work  of  man  is  exam- 
ined with  a  microscope,  the  artist  is  ashamed  of 
the  coarseness  of  his  production  ;  but  no  micro- 
scope is  suflSciently  powerful  to  exhibit  the  minute 
structure  of  the  eye  of  an  elephant  or  a  rhinoceros, 
far  less  of  a  wren  or  an  animalcule. 

In  the  eye  of  man  there  is  marked  care.  It  is 
protected  by  a  projecting  brow,  and  placed  in  such 
a  situation  that  he  can  see  before  him,  beneath 


€4 


STRUCTURE  OF  THE  EYE. 


him,  around  him,  and  above  him.  Its  expressions 
add  much  to  social  intercourse,  and  enable  him 
to  explain  by  a  look  the  thoughts  of  his  heart.  But 
it  is  by  the  reason  with  which  he  is  endowed  that 
his  organs  excel  those  of  every  other  creature. 
When  all  around  is  dark,  he  can  make  artificial 
light.  By  the  aid  of  instruments  which  he  has 
formed,  his  sight  surpasses  that  of  the  lynx  or  the 
eagle,  and  when  it  is  dimmed  by  age,  he  can  re- 
store its  distinctness.  By  the  telescope  he  discov- 
ers that  there  are  no  bounds  to  the  vastness  of  the 
creation,  while  the  microscope  exhibits  that  its 
minuteness  is  unlimited. 

The  more  we  pry  into  the  works  of  the  Al- 
mighty, the  more  do  we  witness  the  design  of  an 
intelligent  contriver.  We  see  with  what  exact- 
ness one  organ  alone  is  made  to  correspond  with 
the  little,  we  know,  of  the  laws  of  optics,  chemis- 
try, and  mechanics.  A  Creator  there  must  be,  who 
is  perfect  in  every  science,  and  in  every  art. 

What  is  the  composition  of  this  masterpiece  of 
design  and  fine  workmanship  ?  The  eye  is  made 
of  the  commonest  materials,  water,  a  little  char- 
coal, some  air,  and  a  few  salts  —  all  ingredients  of 
the  dust  of  the  ground. 

What  is  nature  ?  Is  it  chance  ?  Could  chance 
put  a  rope  over  a  pulley  and  place  it  where  requir- 
ed ?    Could  chance  make  a  telescope  with  glasses 


CONCLUSION.  65 

to  correspond,  or  fit  them  into  a  case  in  which  they 
could  be  regulated  according  to  the  distance  ? 
Above  all,  could  chance  make  the  eye  ?  Could 
chance  prepare  its  magnifiers,  so  clear,  and  trans- 
parent, from  substances,  the  solid  pan  of  which 
is  almost  altogether  charcoal  ?  Could  chance, 
from  the  same  brittle,  porous  material,  make  the 
fibres  of  the  case  so  tough  that  the  fingers  cannot 
pull  it  asunder,  or  weave  them  so  closely  together 
as  to  contain  water?  Could  chance  spin  the  fibres 
of  the  nerve  so  fine,  that  their  minuteness  has  not 
been  ascertained  by  the  microscope,  or  spread 
them  out  at  the  proper  distance  behind  the  magni- 
fiers 1,  Could  chance  make  the  crystalline  lens  of 
a  fish  suited  to  the  element  in  which  it  lives,  or 
prepare  an  apparatus  to  regulate  it  ?  Could 
chance  make  the  iris  a  measurer  of  light  ?  Could 
chance  give  the  eye  so  many  muscles  to  direct  it, 
or  make  the  string  of  the  upper  oblique  to  pass 
through  a  pulley,  and  then  to  go  back,  and  be  fix- 
ed at  the  part  of  the  case  proper  for  rolling  the  eye 
in  the  direction  which  is  wanted  ?  Could  chance 
lodge  the  gland  which  prepares  the  tears  in  a  hol- 
low of  the  bone,  to  be  out  of  the  way  of  the  mo- 
tions of  the  eye,  or  make  the  little  holes  which 
pour  out  the  matter  that  keeps  them  from  running 
on  the  cheek,  or  prepare  the  passages  which  con- 
vey them  away  ?  The  answer  must  be  —  No. 
6* 


66 


STRUCTURE  OF  THE  EYE. 


Of  all  the  creation,  man  alone  has  been  permit- 
ted to  witness  the  magnitude,  the  minuteness,  and 
the  design  which  the  Creator  has  been  pleased  to 
exhibit.  Surely  it  is  not  in  vain  that  he  has  been 
endowed  with  knowledge  to  recognise  Him  in 
His  works. 


MEDICINES  AS  SUBSTITUTES  FOR  SPECTACLES. 

From  the  generally  received  opinion  that  the 
eye  is  adjusted  to  different  distances  by  the  pres- 
sure of  the  external  muscles,  it  is  almost  univer- 
sally stated  that  nearsightedness  is  owing  to  un- 
usual convexity,  and  that  more  remote  vision,  as 
age  advances,  is  occasioned  by  increasing  flatness 
of  the  anterior  surface. 

Though  unusual  convexity  of  the  anterior  sur- 
face, is  an  occasional  cause  of  nearsightedness, 
it  cannot  be  observed  in  a  majority  of  instances. 
As  far  as  my  observation  extends,  the  eye  is  sel- 
dom flattened  by  age,  but  on  the  contrary,  it  is 
often  of  unusual  convexity,  in  consequence  of  the 
contraction  of  a  portion  of  the  circumference 
which  forms  a  semi-opaque  ring,  weW  knov^^n  to 
oculists  under  the  name  of  arcus  senilis ;  yet  the 
persons  thus  affected  are  obHged  to  wear  convex 
spectacles. 

The  true  method  by  which  the  eye  is  adjusted, 
is  clearly  proved,  not  only  by  the  structure  of  the 
organ  in  man,  and  the  lower  animals,  but  also  by 
pliysiological  observatipn.    As  the  apparatus  by 
67 


68 


MEDICINES  AS 


which  the  change  is  effected,  becomes  stiff  with 
age,  the  magnifier  cannot  be  brought  sufficiently- 
far  forward,  to  adapt  the  eye  to  the  vision  of  near 
objects,  and  the  person  is  obliged  to  remedy  the 
defect  by  using, glasses.  Grief,  or  any  other  cause 
that  debilitates  the  body,  also  affects  the  sight. 
There  are  some,  who  though  obliged  to  wear 
glasses  at  their  business,  can  easily  see  to  read 
without  them,  after  leaving  town  for  a  few  days, 
and  enjoyiug  the  invigorating  air  of  the  country. 
On  the  other  hand,  when  the  strength  is  renewed 
by  nourishing  food,  by  wine  or  exhilarating  com- 
pany, the  sight  becomes  improved.  Dr.  Tully, 
the  distinguished  professor  of  Yale  college,  has 
ascertained  that  when  aged  persons  are  under 
the  influence  of  strychnine,  they  can  read  with- 
out their  spectacles.  On  the  other  hand,  it  has 
been  long  known,  that  there  are  medicines  call- 
ed  stramonium,  deadly  nightshade,  and  henbane, 
the  effect  of  which  is  to  relax  the  adjusting  appa- 
ratus, and  thus  accommodate  the  eye  to  the  vision 
of  distant  objects. 

The  use  of  spectacles  should  be  delayed  as  long 
as  possible,  as  when  once  commenced,  it  is  diffi- 
cult to  see  clearly  without  them,  and  permanent 
injury  to  the  eyes  is  often  caused  by  magnifiers  of 
a  high  power.  The  wse  of  spectacles  might  be 
postponed,  by  exerting  the  organs  as  little  as  pes- 


SUBSTITUTES  FOR   SPECTACLES.  69 

sible,  during  the  evening  by  candlelight,  and  to 
give  them  immediate  repose  when  they  become 
hot  and  fatigued. 

The  eyes  may  be  strengthened  by  the  applica- 
tion of  cologne-water,  or  camphorated  spirits,  to 
the  brow  or  temple,  three  or  four  times  a  day,  after 
bathing  the  eyes  with  cold  water.  If  no  effect  is 
produced  by  these  applications  to  the  brow  and 
temple,  they  may  be  changed  for  a  solution  of 
strychnine,  in  equal  quantities  of  alcohol  and  vin- 
egar in  the  proportion  of  two  or  three  grains  to  an 
ounce  of  the  mixed  liquid.  The  medicine  may  be 
also  taken  internally,  in  very  small  doses,  but  it 
should  only  be  used  under  the  direction  of  a  skil- 
ful practitioner,  as  it  is  as  dangerous  in  an  over- 
dose, as  morphine,  iodine,  or  other  powerful  rem- 
edies, which  are  perfectly  harmless,  when  the 
quantity  is  properly  regulated. 

When  glasses  are  indispensible,  the  weakest 
only  should  be  first  selected,  as  other  magnifiers 
will  be  afterward  required. 


70 


MEDICINES  A8 


Dr.  Kitchiner  has  constructed  the  following 
table  of  the  focal  lengths  of  the  convex  or  magni- 
fying-glasses  comnfionly  required  at  various  ages. 


Years  of  Age. 

Focal  Lengths  in  inches. 

40 

36 

45 

30 

50 

24 

55 

20 

58 

18 

60 

16 

DO 

1  A 

14 

70 

12 

75 

10 

80 

9 

85 

8 

90 

7 

100 

6 

Shortsightedness  is  far  more  common  among 
the  higher,  than  among  the  lower  classes  of  soci- 
ety, and  most  frequently  occurs  about  the  time  of 
puberty — a  period  when  there  is  often  a  morbid 
excitement  of  all  the  erectile  tissues.  As  a  por- 
tion of  the  adjusting  apparatus  of  the  eye  belongs 
to  this  class,  it  partakes  of  the  general  erethism  of 


SUBSTITUTES  FOR  SPECTACLES-  71 


the  system,  and  occasions  the  complaint  by  draw- 
ing the  principal  magnifier  too  far  forward. 

By  the  imprudent  commencement  of  the  use  of 
spectacles,  the  magnifier  is  kept  in  a  wrong  posi- 
tion, and  the  efforts  of  nature  to  relieve  the  com- 
plaint are  prevented.  The  adjusting  apparatus 
loses  its  power,  just  as  other  portions  of  the  body 
become  weak  from  want  of  exercise. 

Between  the  ages  of  fourteen  and  eighteen, 
when  the  nearsightedness  is  owing  to  fulness  of 
the  system,  every  effort  should  be  made  to  invigo- 
rate the  health,  by  exercise  in  the  open  air,  cold 
bathing,  the  shower-bath,  agreeable  company, 
proper  wholesome  food,  and  if  necessary,  mild 
aperient  medicine.  All  stimulating  food  and  drinks 
should  be  avoided.  Novel  reading  is  weakening 
by  the  confinement  it  occasions,  as  well  as  by  un- 
due exertion  of  the  sight,  extremely  injurious. 
Shortsightedness  is  seldom  seen  in  a  country-girl, 
while  it  is  frequently  witnessed  among  the  females 
of  a  crowded  city. 

Four  or  six  leeches  may  be  applied  to  each  tem- 
ple every  week,  or  every  two  weeks,  and  the  brow 
and  temples  bathed  with  bay  rum,  night  and  morn- 
ing. The  adjusting  apparatus  may  be  relaxed, 
by  rubbing  over  the  eyebrows,  at  bed-time,  a  very 
small  portion  of  the  extracts  of  belladonna,  stra- 
monium, or  hyosciamus ;  retaining  it  on  the  fore- 


72 


MEDICINES  AS 


head  by  a  bandage,  and  washing  it  off  in  the  morn- 
ing. Small  quantities  of  the  most  suitable  of 
these  remedies,  (hyosciamus,)  may  also  be  taken 
inwardly,  but  they  should  never  be  pushed  so  far 
as  to  cause  much  dilatation  of  the  p\ipil. 

Like  strychnine,  these  medicines  are  unsafe  in 
the  hands  of  those  unacquainted  with  the  practice 
of  medicine ;  besides  as  usually  sold,  they  are 
often  from  age,  or  improper  preparation,  totally 
inert. 


THE  PROXIMATE  CAUSE 


OF 

AFFECTIONS  OF  THE  RETINA. 

Read  before  the  New  York  Medical  and  Surgical  Society, 
October  21,  1837. 

The  spots  which,  witaout  external  existence, 
appear  before  the  eyes,  are  either  fixed  or  floating, 
and  from  the  descriptions  of  patients,  they  assume 
a  great  variety  of  forms.  To  some,  they  appear 
as  an  insect's  wing,  a  spider  with  long  projecting 
legs,  a  network,  a  branch,  or  an  angular  or  straight 
line.  By  others,  they  are  represented  as  twisted 
or  undulating  hairs,  a  string  of  beads,  a  shower  of 
opaque  or  transparent  globules  separate  or  united; 
or  as  tufts  of  black  cotton,  or  the  black  particles 
which  are  produced  from  smoke  and  float  in  the 
atmosphere.  They  are  more  distinctly  observed 
when  the  eyes  are  exposed  to  intense  light,  as 
looking  long  at  white  objects  well  illuminated,  at 
flame,  or  at  a  clear  sky. 

Various  opinions  have  been  entertained  about 
the  nature  of  muscce  volitantes.  Morgagni  sup- 
7 


74 


THE  PROXIMATE  CAUSE  OF 


posed  them  to  arise  from  thickened  lachrymal  hu 
mour,  formed  on  the  cornea.  On  account  of  their 
motion,  Delahire  thought  they  arose  from  substan- 
ces floating  in  the  aqueous  humour,  and  to  support 
his  opinion,  made  the  following  experiment :  He 
received  upon  white  paper,  the  rays  of  the  sun 
through  a  pane  of  glass,  in  which  there  were  ves- 
icles and  filaments  and  the  imperfections  in  the 
glass  appeared  upon  the  paper,  as  the  bodies  in 
question  upon  the  retina.* 

To  ascertain  if  Delahire's  opinion  was  correct, 
Demours  opened  the  cornea  and  evacuated  the 
aqueous  humour,  as  had  been  proposed  by  Le 
Roy,  without  the  least  diminution  of  the  appear 
ances.  He  consequently  concluded  that  the  seat 
of  the  disease,  was  the  humour  of  Morgagni,  some 
small  portions  of  which,  without  loss  of  transparen- 
cy, had  acquired  a  greater  density  and  refrangi- 
bility. 

As  it  is  obvious  that  the  light  refracted  by  the 
lenses  of  the  eye  and  received  upon  the  retina  can- 
not be  compared  to  the  passage  of  light  through  a 
pane  of  glass,  I  modified  Delahire's  experiment  by 
using  a  double  convex  lens  which  contained  a 
number  of  strict.  I  found  that  the  shadows  of 
these  stricR  were  visible  upon  a  screen  at  a  certain 


*  Demours. 


AFFECTIONS   OF  THE  RETINA. 


75 


distance  from  the  glass,  but  at  situations  far  be- 
yond its  focus.  When  the  screen  was  moved  to- 
wards the  focus,  the  inverted  picture  was  not  in 
the  least  interrupted  nor  were  shadows  produced 
on  the  picture  by  placing  small  bodies  before  or 
immediately  behind  the  glass.  Hence  we  may 
conclude  that  spots  or  filaments  in  the  crystalline 
lens,  or  anterior,  or  immediately  posterior  to  it, 
camiot  be  represented  on  the  retina- 
Most  people  in  ophthalmic  practice,  have  seen 
small  fragments  of  capsule  remaining  in  the  pupil 
after  the  operation  for  cataract,  and  have  found  on 
enquiry  that  vision  was  not  disturbed  by  a  fixed 
or  flying  musca,  nor  are  these  symptoms  produ- 
ced by  small  ulcers  on  the  cornea.  A  case  occur- 
red to  Mr.  Ware,  in  which  after  the  removal  of  a 
cataract,  a  white  opaque  particle,  about  the  size  oi 
the  head  of  a  small  pin,  moved  continually  up- 
ward and  downward  near  the  centre  of  the  pupil. 
Though  very  perceptible  to  observers,  it  was  whol- 
ly unperceived  by  the  patient,  and  neither  inter- 
fered with  vision  nor  occasioned  the  slightest  ap- 
pearance of  a  musca  volitans.  I  have  myself  op- 
erated for  the  removal  of  a  fragment  of  capsule, 
which  floated  in  the  pupil,  and  though  previous  to 
the  last  operation,  the  vision  was  partially  cloudy 
and  wavering,  still  there  was  no  appearance  that 
might  be  compared  to  those  under  consideration. 


76 


THE   PROXIMATE   CAUSE  OF 


Guided  in  his  opinion  by  the  principles  of  optics, 
Pitcairn  thought  the  disease  arose  from  congestion 
or  varicosity  of  some  of  the  bloodvessels  of  the 
retina.*  By  Willis  it  was  attributed  to  insensibility 
of  certain  filaments  of  the  optic  nerve. 

"The  fixed  musca,"  says  Mr.  Travers,  "is  gen- 
erally an  organic  affection,  probably  a  deposite  or 
extravasation  between  the  choroid  and  retina,  com- 
pressing to  a  certain  space  the  papillae  of  the  retina, 
to  which  the  musca  corresponds  in  figure.  In 
other  instances  it  is  independent  of  deranged  struc- 
ture and  may  be  presumed  to  be  an  insensible  spot 
of  the  retina." 

In  his  memoir  upon  the  subject,  published  in  the 
fifth  volume  of  the  Medico-Chirurgical  Transac- 
tions, Mr.  Ware  states  that  "  it  is  not  easy  to  as- 
certain the  proximate  cause  of  these  moats,  but 
from  the  constancy  in  their  figure,  and  their  fre- 
quently long  continuance,  it  seems  probable  that 
they  depend  upon  a  steady  pressure  on  one  or 
more  minute  points  of  the  retina,  which  are  situa- 
ted near  the  axis  of  vision,  but  not  exactly  in  it. 
The  pressure  must  be  near  this  axis,  because  the 
moats  always  appear  near  the  objects  that  are 
looked  at ;  but  it  cannot  be  in  the  axis,  because  the 
moats  do  not  injure  or  impair  their  natural  appear- 


♦  Demours, 


AFFECTIONS   OF   THE  RETINA. 


77 


ance.  As  the  pressure  is  not  in  the  axis,  the  out- 
lines of  the  moats  is  always  somewhat  obscure, 
and  the  exertion  that  is  made  to  bring  the  moats 
into  the  axis,  by  moving  the  eye  gives  them  an 
apparent  motion,  which  is  sometimes  upward  and 
downward,  and  sometimes  from  side  to  side.  That 
the  tunica  retina  is  Hable  to  be  affected  by  this  par- 
tial pressure,  may  be  fairly  inferred  from  an  exam- 
ination of  the  structure  of  this  tunic  in  connection 
with  the  parts  that  are  contiguous  to  it." 

"  The  concave  surface  of  the  choroides,  when 
well  injected,  has  been  said  by  Tina,  to  have  a 
villous  appearance,  produced  by  innumerable  short 
flocculi  which  are  exquisitely  minute,  and,  indeed, 
they  are  perceptible  to  the  naked  eye-  They  are 
covered  by  a  black  mucous  substance,  called  the 
pigmentum  nigrum,  which  is  so  equally  spread 
over  the  retina,  that  when  the  person  is  in  health, 
it  only  serves  to  render  the  retina  duly  suscepti- 
ble of  the  impressions  made  upon  it  by  the  light 
transmitted  from  external  objects.  When,  how- 
ever, a  morbid  sensibility  is  excited,  like  that  which 
general  debility,  or  much  anxiety,  is  apt  to  occasion, 
the  retina  (which  has  a  larger  quantity  of  nervous 
medulla  spread  over  it  in  proportion  to  its  dimen- 
sions, than  any  other  part  of  the  body)  becomes 
morbidly  impressed  by  any  little  points  or  projec- 
7* 


78 


THE  PROXIMATE   CAUSE  OP 


lions  that  happen  to  be  in  contact  with  it.  This 
morbid  impression  may  be  occasioned  either  by 
the  pressure  of  small  portions  of  lymph,  diffused 
irregularly  between  the  choroid  coat  and  retina ; 
by  some  minute  particle  of  the  pigmentum  nigrum 
larger  or  more  uneven  than  the  rest,  or  by  one  or 
more  of  the  minute  villi  of  the  choroides  itself ; 
and  such  a  pressure,  however  it  be  occasioned,  is 
sufficient  in  my  apprehension,  to  produce  the  im- 
age of  an  object,  similar  in  every  respect,  to  that 
of  a  real  object  so  situated  that  light  proceeding 
from  it  would  have  produced  a  similar  impression 
upon  the  retina.  The  imaginary  object  is  seen  in 
a  right  line,  continued  from  the  point  where  the 
impression  is  made  on  the  retina,  through  the  cen- 
tre of  the  eye,  conformably  to  a  known  law  in  op- 
tics ;  and  the  distance  at  which  it  is  seen  from  the 
eye  is  that  at  which  objects  of  a  similar  size  are  in 
general  most  distinctly  perceived.  The  difference 
between  the  structure  of  the  retina  and  choroides, 
when  capable  of  producing  these  morbid  sensa- 
tions, and  that  of  these  parts  when  in  perfect  health, 
is,  however,  exquisitely  minute  ;  and  the  morbid 
impressions  made  on  the  retina,  are  so  much  out 
of  the  line  of  the  axis  of  the  eye,  that  the  imagin- 
ary moats  they  occasion,  do  not  interfere  with  the 
sight  of  external  objects,  these  moats  being  in  gen- 
eral so  faint  and  undefined,  that  they  can  only  be 


AFFECTIONS   OF  THE  RETINA.  79 


perceived  when  the  light  is  strong,  and  the  atten- 
tion is  directed  particularly  to  them." 

As  it  is  only  by  a  knowledge  of  structure  that 
we  can  acquire  a  philosophical  view  of  disease,  we 
shall  examine  the  descriptions  of  the  retina,  by  the 
authors  most  frequently  perused. 

Mr.  Ware,  in  the  paper  just  quoted,  says,  "  the 
retina  in  a  recent  human  eye,  has  the  appearance 
of  a  plain,  uniform,  transparent,  pulpy  membrane, 
which  surrounds  the  vitreous  humour,  but  is  un- 
connected with  it.  On  a  close  examination  it  is 
discovered  to  be  composed  of  two  substances.  One 
of  these  is  an  exquisitely  thin  membrane,  on 
the  inner  side  of  which,  in  the  foetal  subjeci,  many 
bloodvessels  may  be  traced,  and  on  the  outer  a 
medulla  is  spread,  which  lies  in  contact  with  the 
inner  concave  surface  of  the  tunica  choroides" 

Sir  Charles  Bell,  in  his  Bridgewater  Treatise, 
which  is  his  latest  publication,  says,  that  "  there  is 
no  fibrous  texture  in  the  matter  of  the  nerve," 
(meaning  the  retina,)  and  again  when  combating 
Herschel's  hypothesis  of  vision,  "  It  appears  to 
me  natural  to  suppose  that,  if  these  fibres  of  the 
nerve  {which,  be  it  remembered,  are  also  imagin- 
ary) were  moved  like  the  cords  of  a  musical  in- 
strument, they  would  be  most  easily  continued  in 
motion  by  undulations  in  the  same  time,"  &c. 

The  most  modern  work  on  the  structure  of  the 


80  THE   PROXIMATE   CAUSE  OP 

eye,  which  I  have  seen,  is  that  of  Dalrymple,  pub- 
hshed  in  1834.  His  account  of  the  retina  is  as 
follows : — 

"  Several  of  the  older  anatomists,  among  whom 
we  find  the  names  of  Ruysch  and  Briggs,  assert 
that  the  retina  is  composed  of  minute  fibres,  evi- 
dently derived  from  the  fibrous  appearance  of  the 
optic  nerve,  and  thence  radiating  in  the  form  of  a 
star.  Haller  denies  this,  and  attributes  the  ap- 
pearance in  question,  to  certain  plicae  or  folds,  into 
which  the  retina  probably  falls,  by  the  evaporation 
or  dispersion  of  the  fluid  of  the  vitreous  body  after 
death.  It  has  assuredly  never  happened  to  me  to 
observe  any  fibres,  or  any  appearance  that  could 
be  mistaken  for  fibres  in  a  recent  human  eye." 

Let  us  now  appeal  to  nature  for  the  correctness 
of  these  assertions. 

When  the  inner  surface  of  the  choroid  is  exam- 
ined with  a  microscope,  in  place  of  the  villi  of 
Tina  and  Ware,  we  find  the  membrane  of  Mondi- 
ni,  upon  which  is  deposited  the  globules  of  the 
pigmentum  nigrum. 

When  the  convex  surface  of  the  retina  is  ex- 
posed under  water,  and  scratched  with  a  scalpel,  a 
membrane  of  great  delicacy  may  be  separated  and 
turned  over  in  folds,  with  the  assistance  of  a 
camel's-hair  pencil.    This  is  the  coat  of  Jacob. 

When  the  same  preparation  is  allowed  to  putre- 


AFFECTIONS  OF  THE  RETINA. 


81 


fy,  and  the  nervous  matter  washed  away  with  a 
camel's-han-  pencil,  the  vascular  membrane  may 
be  exhibited.  The  ramifications  of  the  blood-ves- 
sels on  this  membrane,  resemble  that  of  the  veins 
in  a  leaf,  after  the  soft  part  has  been  eaten  away 
by  insects,  and  by  their  intertexture,  they  form  a 
semi-opaque  screen,  on  which  is  received  the 
image  of  external  objects,  just  as  the  ground 
of  a  camera-obscura,  or  the  screen  of  a  magic- 
lantern. 

The  nervous  matter  may  be  divided  into  two 
layers.  By  allowing  an  eye  to  macerate  in  alco- 
hol, for  the  purpose  of  preventing  the  retina  from 
collapsing,  when  the  anterior  half  of  the  eye  is 
cut  off,  and  pouring  upon  the  retina  thus  exposed, 
a  watery  solution  of  corrosive  sublimate,  the  fibres 
may  be  seen  lying  beneath  the  vascular  mem- 
brane, when  they  are  separated  by  a  camel's-hair 
pencil.  In  young  animals,  the  fibres  are  more 
easily  exhibited  than  in  those  that  are  old,  and  in 
the  human  eye  they  converge  round  the  central 
foramen.  By  pouring  upon  an  eye,  exposed  in  the 
same  manner,  an  alcoholic  solution  of  corrosive 
sublimate  and  muriate  of  ammonia,  the  fibrous 
coat  becomes  so  compact  and  hard,  that  it  may  be 
easily  torn  off  with  forceps,  and  a  layer  of  glob- 
ules will  be  brought  into  view.  These  globules 
are  kept  in  place  by  the  coat  of  Jacob,  which  is 


82 


THE   PROXIMATE   CAUSE  OF 


reflected  over  the  choroid,  as  may  be  easily  seen 
by  opening  an  eye  under  alcohol. 

The  retina  then  consists  of  four  layers,  a  vas- 
cular, a  fibrous,  a  globular,  and  a  serous. 

By  this  exposition  of  the  retina,  we  may  ac- 
count for  the  various  appearances  of  musccB  voli- 
tantes.  I  have  occasionally,  w^hen  entering  an  or- 
dinarily lighted  room,  after  a  full  meal,  and  expo- 
sure to  a  bright  light,  witnessed  glimmerings  like 
a  network,  which  from  its  resemblance  to  the  vas- 
cular coat,  left  no  doubt  in  my  mind,  that  the  blood- 
vessels of  the  retina  were  visible  :  at  other  times 
in  the  same  circumstances,  there  was  a  twisted 
tube,  or  a  chain  of  beads,  as  if  there  had  been  an 
error  loci  of  one  of  the  curved  fibres  of  the  retina ; 
or  there  was  a  cloud  of  globules  sometimes  packed 
together,  but  more  frequently  separated,  and  float- 
ing in  all  directions.  Each  globule  was  visible  for 
a  considerable  time,  and  repeatedly  reoccupied  the 
same  space.  When  clustered  together,  they  had 
a  great  resemblance  to  the  globules  of  the  re- 
tina. 

From  the  similarity  of  the  drawing  of  the  float- 
ing network,  in  Case  No.  1,  to  the  vascular  coat 
of  the  retina,  I  am  persuaded  that  any  person  who 
has  seen  both,  will  have  no  hesitation  in  locating 
the  disease ;  and  if  the  net-work,  curved  filaments, 
and  globules,  appear  to  others  as  they  do  to  me, 


AFFECTIONS  OF  THE  RETINA.  83 

the  various  musccB  will  be  ascribed  to  affections 
of  the  structure  which  they  resemble. 

Beer  observed  in  some  cases,  that  the  vessels  of 
the  vascular  membrane  had  become  varicose.  Dr. 
Wardrop,  in  his  "  Morbid  Anatomy  of  the  Eye," 
states  that  "  it  was  observed  by  Sauvages,  that  the 
pulsations  of  the  optic  artery  might  be  perceived, 
by  looking  intently  at  a  white  wall,  well  illumina- 
ted. A  kind  of  network,  darker  than  the  other 
parts  of  the  wall,  appears  and  vanishes  at  every 
pulsation.  This  change  of  color  in  the  wall,  he 
ascribed  to  the  compression  of  the  retina,  by  the 
diastole  of  the  artery.  Richter  mentions  the  case 
of  a  plethoric  person,  who,  when  he  held  his 
breath,  and  looked  at  a  white  wall,  perceived  a 
kind  of  network,  which  alternately  appeared  and 
disappeared  with  the  diastole  and  systole  of  the 
arteries.  Mr.  George  Young,  saw  a  cadet  at 
Woolwich,  who,  from  being  obliged  to  wear  a  very 
tight  neckcloth  and  collar,  had  his  sight  very  much 
impaired  ;  the  pupils  were  dilated,  and  he  had  the 
appearance  of  flies  floating  constantly  before  his 
eyes  ;  and  Richter  relates  the  case  of  a  man,  who 
became  suddenly  blind,  by  carrying  a  heavy  load 
up  stairs." 

If  when  the  eye  is  directed  forward  at  a  distance, 
we  move  a  lighted  candle  up  and  down  on  one 
side  of  the  line  of  vision,  a  representation  of  the 


84 


THE   PROXIMATE   CAUSE  OF 


vessels  of  the  vascular  membrane  shortly  appears, 
as  if  displayed  upon  a  screen.  The  vessels  are 
greatly  magnified,  on  account  of  the  portion  of  the 
retina  vi^hich  they  occupy,  compared  with  that  of 
an  ordinary  image.  We  may  hence  infer,  that  a 
very  minute  congestion,  may  cause  a  large  musca. 

It  is  stated  by  Demours,  that  the  diameter  of 
muscse  appears  to  increase  in  proportion  as  we  re- 
cede from  the  plain  in  which  they  are  examined. 
Such  a  filament  as  appears  one  sixth  of  a  line  in 
diameter,  and  one  inch  long,  when  seen  on  a  leaf 
of  very  white  paper,  at  the  usual  focal  distance, 
appears  two  lines  in  diameter,  and  more  than  a 
foot  long,  when  we  examine  it  by  looking  at  a 
white  wall,  at  the  distance  of  twenty  or  thirty  feet, 
and  in  the  only  case  in  which  he  mentions  the 
subject,  Mr.  Ware  says,  that  the  magnitude  of  the 
moats,  depended  much  on  the  distance  at  which  they 
were  observed,  being  larger  when  seen  far  off,  and 
smaller  when  near  the  eyes.  Can  the  diminution 
arise  from  the  less  degree  of  pressure  as  the  lens 
approaches  the  cornea,  when  adjusted  to  near  ob- 
jects ? 

As  the  papillae  of  other  nerves,  become  erected 
when  excited,  it  would  seem  that  the  fifth,  which 
is  a  compound  nerve,  enables  the  expanded  fibres 
of  the  optic  nerve,  to  be  placed  in  a  proper  condi- 
tion for  conveying  a  distinct  impression  to  the  sen- 


AFFECTIONS  OF  THE  RETINA.  85 

sorium.  Should  there  be  any  unusual  lurgescence 
of  the  vascular  membrane,  or  any  error  loci^  of 
the  globules,  or  of  the  sentient  fibres,  or  diminish- 
ed supply  of  motive  povi^er  to  the  latter,  or  should 
there  be  effusion  of  lymph  or  varicosity  of  the 
choroid,  the  fibres  v^^ill  not  be  free  to  the  action  of 
light,  but  will  convey  false  impressions,  and  there 
will  be  an  appearance  of  motion,  when  during 
their  erection  or  tension,  the  fibres  come  in  con 
tact  with  diseased  vessels,  filaments,  or  globules. 

It  is  difl^icult  to  keep  the  eye  on  one  object  for 
a  long  time,  but  when  it  is  accomplished,  the  reti- 
na soon  becomes  fatigued,  the  fibres  lose  their 
lone  or  tension,  and  the  object  disappears.  As 
soon  as  they  have  rested,  the  object  comes  again 
into  view,  and  there  is  an  alternate  disappear- 
ance and  reappearance  of  the  object,  as  long  as 
the  experiment  can  be  continued.  If  when  the 
light  is  very  obscure,  we  look  intently  at  a  feebly 
illuminated  object,  the  fibres,  in  endeavouring  to 
adapt  themselves  to  the  degree  of  light,  soon  be- 
come painfully  affected,  and  the  object  is  no  longer 
visible. 

The  connection  between  the  second  and  the  fifth 
pair  of  nerves,  may  explain  why  there  is  a  halo 
round  luminous  objects  during  catarrhal  ophthal- 
mia, when  the  distended  vessels  press  upon  the 
filaments  of  the  latter,  which  are  so  abundantly 


86 


THE   PROXIMATE   CAUSE  OF 


spread  upon  the  conjunctive  ;  and  why  in  strum- 
ous ophthalmia,  there  is  such  intolerance  of  light, 
when  the  nerves  are  irritated  by  exposure  in  con- 
sequence of  ulceration  of  the  anterior  membrane. 

Case  1. — A  highly  intelligent  gentleman  from 
Newark,  N.  J.,  consulted  me  some  time  ago,  about 
a  network  which  appeared  before  his  eyes,  and 
impeded  vision.  While  describing  his  complaint 
he  drew  a  representation  of  part  of  the  vascular 
coat  of  the  retina,  as  perfectly  as  if  he  had  a  pre- 
paration of  the  membrane  before  him  for  a  copy. 
I  took  an  unusual  interest  in  the  case  from  the 
drawing,  and  obtained  the  following  account  of  it. 


The  appearances  before  my  eyes  are  drawn  as 
correctly  as  I  am  able ;  they  do  not  appear  station- 
ary, for  when  I  suddenly  throw  my  eyes  up,  they 
will  also  go  up,  and  appear  to  rise  above  the  ob- 
ject upon  which  I  fix  my  sight ;  they  then  move 
slowly  downward  and  sink  below  the  sight,  some- 
times a  little  on  one  side  and  at  other  times  ex- 
actly in  the  way,  so  as  to  cover  a  letter  or  figure 
at  which  I  may  happen  to  look,  I  can  still  see  the 


AFFECTIONS  OF  THE  RETINA. 


87 


object,  though  imperfectly,  as  if  through  thin  gauze 
or  something  of  the  kind,  which  dims  the  sight  a 
little.  When  they  get  on  one  side  of  the  sight, 
and  I  attempt  to  turn  my  eyes  to  get  a  more  com- 
plete view  of  them,  turning  my  eyes  appears  to 
turn  them  also,  and  they  keep  at  about  the  same 
distance.  When  my  eyes  are  open  they  do  not 
appear  so  large,  nor  so  plainly  as  they  do  when  I 
partly  close  my  eyes.  When  I  fix  my  sight  upon 
an  object  and  look  at  it  for  a  moment  or  two  with- 
out moving  my  eyes,  they  will  disappear,  but  the 
least  motion  of  the  eyes  will  bring  them  back 
again  as  plainly  as  ever.  I  have  two  or  three  dif- 
ferent times,  noticed  a  kind  of  motion  as  if  caused 
by  hundreds  of  small  insects  in  strong  daylight. 
The  distance  of  the  network  appearances  seems  to 
be  regulated  by  the  distance  of  the  objects  at  which 
I  look.  If  I  look  at  an  object  half  a  mile  off  they 
seem  considerably  farther  from  me,  than  they  do 
when  I  look  at  an  object  which  is  only  two  or  three 
feet  distant.  The  appearances  are  worse  after  a 
hearty  dinner,  or  loss  of  sleep,  and  they  trouble 
me  least  when  I  feel  otherwise  well. 

As  there  appeared  considerably  bilious  derange- 
ment, I  prescribed  an  emetic  and  alterative  doses  of 
calomel.  I  also  recommended  regular  cupping 
every  two  weeks,  cold  applications  to  the  brow  and 
temple,  a  loose  neckcloth,  and  to  avoid  whatever 


88 


AFFECTIONS  OF  THE  RETINA. 


might  occasion  straining.  He  is  now  much  im- 
proved, though  the  network  is  occasionally  visible. 

Case  2. — The  studies  of  a  young  gentleman  at 
Princeton  College,  were  interrupted  by  a  network 
which  was  without  motion,  and  which  was  always 
present  and  impeded  vision.  The  regular  appli- 
cation of  leeches  was  recommended  but  they  were 
not  applied.  Cold  applications  and  aperients  had 
no  effect.  Mercury  and  Iodine  were  tried  without 
benefit.  The  loss  of  blood  was  again  urged,  and 
the  patient  was  immediately  relieved  after  the  ap- 
plication of  cupping-glasses  to  the  temple.  By 
weekly  repetition  of  the  cupping  he  soon  became 
quite  well. 

Case  3. — A  theological  student  from  Schenec- 
tady, consulted  me  about  hundreds  of  moving  glob- 
ular specks  which  without  any  other  morbid  symp- 
tom, were  constantly  floating  before  his  eyes,  and 
caused  much  inconvenience  and  anxiety.  He  was 
not  at  all  improved,  after  having  made  use  of  vari- 
ous remedies. 

Case  4. — A  gentleman  was  for  upward  of  a  year, 
annoyed  by  a  floating  musca  similar  to  a  portion 
of  the  soot  produced  by  burning  turpentine,  and 
which  flies  in  the  air.  After  having  had  occasion 
to  undergo  a  course  of  mercury  for  another  com 
plaint  he  was  entirely  cured. 


